Page 237 – Clinical Laboratory int.

NGAL as a biomarker of acute kidney injury

, 26 August 2020/in Featured Articles /by 3wmedia

The diagnosis of acute kidney injury (AKI) is often hindered by the reliance on serum creatinine as a marker of kidney function, which can delay detection. Neutrophil gelatinase-associated lipocalin is a promising biomarker which increases within hours of kidney damage and could therefore improve the early diagnosis of AKI.

by Dr Ashley Garner

Clinical background
Acute kidney injury (AKI) is a common condition associated with significant morbidity and mortality. It is currently diagnosed using serum creatinine and urinary output as markers of kidney function, as defined in the recent KDIGO criteria [Fig. 1][1]. However, these are relatively late markers of AKI since they mainly reflect a decrease in glomerular filtration rate and the time required for serum creatinine to accumulate can delay diagnosis. Biomarkers that can detect structural injury to the kidney rather than a loss of function may allow better and earlier detection of AKI. Earlier diagnosis of AKI could facilitate earlier intervention, potentially reduce the risk of irreversible kidney damage and improve patient outcomes. Much research in recent years has therefore focused on the discovery of improved biomarkers for AKI and neutrophil gelatinase-associated lipocalin (NGAL) is one of the most promising candidates [2].

Pathophysiology of NGAL
NGAL is a small 25kDa protein which belongs to the superfamily of lipocalins. It is expressed in many cells including neutrophils, hepatocytes and renal tubular cells and is induced in response to pathological stimuli including infection, inflammation, ischemia and malignancy. NGAL has a functional role in the innate immune system as a bacteriostatic agent, depleting iron-binding siderophores and thereby preventing bacterial iron acquisition. The iron-binding properties of NGAL are also proposed to provide protection from oxidative stress. There is growing evidence that NGAL also acts as a growth factor in some tissues including renal epithelial cells where it modulates cell proliferation, differentiation and apoptosis and may provide protection against renal tubular damage in AKI [3].

Animal studies of AKI induced by ischemia or nephrotoxicity have shown that NGAL is one of the most upregulated proteins in the kidney and is detectable in the urine within 2–3 h. It has been reported that urine NGAL concentrations increase 25–100 fold and plasma NGAL increases 7–16 fold following AKI. Unlike serum creatinine, NGAL is not increased when there is impaired glomerular filtration without renal tubular damage, often termed ‘pre-renal’ uraemia.
Low plasma concentrations of NGAL are found in health as it is expressed at a low constant rate from various tissues. NGAL is then freely filtered at the kidney and the majority is reabsorbed in the proximal tubule, resulting in low NGAL concentrations in the urine. Following AKI, NGAL is greatly upregulated in the cells lining the ascending loop of Henle and collecting ducts of the kidney and is then excreted in the urine. The origin of the increase in plasma NGAL following AKI is less clear and there is evidence to suggest that NGAL expression is increased in other organs such as the lungs and liver following kidney injury.

Since NGAL can be produced by different tissues in response to various stimuli, it is not specific to AKI. Other common conditions that can cause elevated NGAL, and therefore complicate the interpretation of results, include sepsis, heart failure, chronic kidney disease (CKD), malignancy and urinary tract infections.

NGAL assays
Commercial CE marked assays are available for measuring NGAL in plasma, whole blood and urine. It is not clear from the literature whether any of these sample types are preferred or have better diagnostic performance but there are limiting factors for each that may require consideration. Plasma and whole blood samples are invasive and may be contaminated by haemolysis releasing NGAL from neutrophils. Urine NGAL may theoretically be more sensitive for AKI due to greater induction in renal tubular cells but can be falsely elevated in urinary tract infections due to leukocyturia and it is still unclear whether the NGAL should be corrected for urine concentration effects or whether this is unnecessary or even misleading in AKI. Although non-invasive, urine samples may be more difficult to obtain especially at specific time points or if the patient has reduced urine output.

NGAL exists in monomeric, dimeric and heterodimeric or complexed forms. It has been reported that the monomer is the predominant form produced by renal tubular cells and the homodimer is predominantly released by neutrophils. The relevance of these different forms of NGAL will depend on the extent to which NGAL assays detect them and the sample type used. Even though the monomer form may be most relevant for urine NGAL the origin of plasma NGAL in AKI is less clear and may therefore include the other forms. This variation in NGAL assays and sample types makes it difficult to directly compare study results and derive clinically relevant cut-off values. Standardization of NGAL assays using an internationally approved reference material would greatly improve this variation but this is not currently available and would require agreement on what forms of NGAL should be measured.

Research from large heterogeneous populations suggests that urine NGAL concentrations are dependent on gender, age and ethnicity. Biological variation for urine NGAL has also been reported to be as high as 84%. These factors will need to be taken into account when establishing reference intervals for NGAL although they may not be clinically significant if a cut-off value is used to diagnose AKI, especially if it greatly exceeds the expected reference intervals in health.

Clinical utility of NGAL in AKI
There is evidence that NGAL could be useful as an early diagnostic and prognostic biomarker for AKI. Many studies have demonstrated that NGAL rises 24–72 h before creatinine in patients with AKI and is associated with poorer outcomes. It is difficult to determine the diagnostic performance of NGAL for AKI in terms of clinical sensitivity and specificity however, due to the limitations of using serum creatinine as the gold standard comparator. For example, the rise in creatinine caused by pre-renal uraemia will not be associated with a raised NGAL. In addition, a multicentre pooled analysis of prospective studies has shown that patients who have raised NGAL without increases in serum creatinine are at increased risk of adverse outcomes and suggests these patients have a condition termed ‘subclinical AKI’ where there may be tubular damage without glomerular impairment [Fig. 2][4].

The majority of studies assessing NGAL testing in AKI have focused on specific patient populations at high risk of AKI: namely post-cardiac surgery, post contrast infusion, intensive care and emergency admissions.
The advantage of using NGAL in post-surgery or post-contrast patients is that the time of insult is known and therefore NGAL can be measured at set time points for the early detection of AKI and timely intervention. It is more difficult to determine the best application of NGAL in ICU patients in regard to the timing and frequency of tests and it is less clear whether earlier detection can improve outcomes in these patients frequently complicated by multi-organ failure. In the emergency admissions setting NGAL has fewer advantages over serum creatinine since early detection (within hours) is less likely to be applicable. Also NGAL, like creatinine, can be raised in CKD so may require multiple measurements to detect AKI but more patients are likely to have had a previous creatinine result than an NGAL result.

Although there is an abundance of observational studies showing that AKI can be detected earlier using NGAL compared to serum creatinine there is an absence of randomized clinical trials to demonstrate that using NGAL instead of current practice will improve patient outcomes or provide cost benefits. This is probably the biggest barrier to the adoption of NGAL testing in routine practice and better treatments and interventions may be required to overcome it. This would suggest that one of the most important roles for NGAL and earlier biomarkers of AKI is in the discovery and development of effective interventions and therapeutics.
Another consideration regarding interventions for AKI is that NGAL only detects renal tubular damage, it does not distinguish between different causes. However, effective treatments may require the underlying cause to be determined and therefore further biomarkers may be needed to differentiate between causative factors and indicate the most appropriate intervention.

Conclusion
A large number of clinical studies suggest that NGAL may provide an early diagnostic and prognostic biomarker for AKI. However, further randomized clinical trials comparing the use of NGAL to standard practice are required to show cost benefits or improvements in patient outcomes. It seems that even if biomarkers like NGAL enable us to detect AKI earlier, this alone may not be sufficient to improve patient care but hopefully they will facilitate the development of better interventions that will eventually lead to improved outcomes for patients with AKI.

References
1. Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int. Suppl. 2012; 2: 1–138.
2. ADQI Consensus on AKI Biomarkers and Cardiorenal Syndromes. Contrib Nephrol. Basel: Karger, 2013; 182: 13–29.
3. Schmidt-Ott KM, Mori K, Li JY, Kalandadze A, Cohen DJ, Devarajan P, Barasch J. Dual action of neutrophil gelatinase-associated lipocalin. J Am Soc Nephrol. 2007; 18: 407–413.
4. Haase M, Devarajan P, Haase-Fielitz A, Bellomo R, Cruz DN, Wagener G, Krawczeski CD, Koyner JL, Murray P, Zappitelli M, Goldstein SL, Makris K, Ronco C, Martensson J, Martling CR, Venge P, Siew E, Ware LB, Ikizler TA, Mertens PR. The outcome of neutrophil gelatinase-associated lipocalin-positive subclinical acute kidney injury: a multicenter pooled analysis of prospective studies. J Am Coll Cardiol. 2011; 57(17): 1752–1761.

The author
Ashley Garner PhD
Department of Blood Sciences,
Leeds Teaching Hospitals Trust, Leeds, UK

E-mail: Ashley.Garner@leedsth.nhs.uk

Fibroblast growth factor-23: an emerging biomarker in chronic kidney disease

, 26 August 2020/in Featured Articles /by 3wmedia

Fibroblast growth factor-23, a key regulator of phosphate and 1,25-dihydroxyvitamin D metabolism, appears to be an independent risk factor for mortality among chronic kidney disease patients. However, sample stability and poor analytical agreement between detection methods still need to be addressed for it to become a reliable biomarker.

by Dr A. Kumar, Dr W. Herrington, Dr S. Clark and Dr M. Hill

The function of fibroblast growth factor-23 and its role in chronic kidney disease
Fibroblast growth factor-23 (FGF-23) was identified as the key regulator of phosphate homeostasis from a study of renal phosphate-wasting condition autosomal dominant hypophosphatemic rickets [1]. It is secreted principally by bone-forming osteocytes and osteoblasts in response to increased dietary phosphate intake and abnormally elevated serum phosphate concentration (hyperphosphatemia). FGF-23 acts to correct raised phosphate levels by increasing urinary phosphate excretion, by direct inhibition of renal tubular phosphate reabsorption, and via reducing dietary phosphate absorption, by suppressing 1α-hydroxylation of 25-dihydroxyvitamin D to form active 1,25-dihydroxyvitamin D [1,25(OH)2D]. Low 1,25(OH)2D production also provides a negative feedback signal in phosphate homeostasis by inhibiting further FGF-23 secretion (Fig.1)[2, 3].

Chronic kidney disease (CKD) commonly causes a fall in glomerular filtration rate resulting in a reduced capacity for phosphate excretion [4]. FGF-23 levels increase early in CKD, often before any detectable rise in phosphate concentration [5] and those with the severest form of CKD, end-stage renal disease (ESRD), have FGF-23 levels that are 100 to 10,000-fold higher than healthy controls [4, 6]. Sustained suppression of renal 1,25(OH)2D synthesis by high FGF-23 levels contributes to lower serum calcium concentration, a stimulant of parathyroid hormone (PTH) secretion. PTH maintains normal serum calcium concentration by promoting reabsorption of the calcium from its reservoir in bone. The abnormal elevated levels of FGF-23 seen in CKD thus results in a disruption of the homeostatic balance of calcium and phosphate and this may impact on many normal processes including bone mineral metabolism and cardiovascular function. The consequences of prolonged derangement of bone mineral metabolism (known as CKD-mineral bone disease; CKD-MBD) is bone pain and increased fracture risk. CKD-MBD may also accelerate calcification of the vascular tree, a process that may explain some of the significantly increased cardiovascular risk in those with CKD [7]. Indeed, among CKD patients, several studies have shown FGF-23 to be an independent risk factor for mortality [8] and among those not on dialysis it appears to also predict CKD progression [4, 9]. Treatments that might positively impact on FGF-23 levels, for example, reducing dietary phosphate absorption with phosphate binders may therefore have beneficial effects on bone, renal and cardiovascular outcomes in those with CKD (both in those with hyperphosphatemia and those with high-normal serum phosphate concentration).

Methods for FGF-23 assessment
In vitro studies have shown that some of the FGF-23 synthesized by the osteocytes is cleaved between amino acid 179 and 180 by furin (a type I precursor convertase) releasing a C terminal fragment. Current immunometric methods detect either ‘intact’ FGF-23 (iFGF-23, ~32 KDa) or ‘C-terminal’ fragments (cFGF-23, ~14 KDa) in plasma or serum. The cFGF-23 assays recognize two epitopes in the C-terminus, thereby recognizing both iFGF-23 and cFGF-23 fragments. The intact assays recognize only the iFGF-23 because the epitopes flank the cleavage site [2]. At present, there is no reference method, or consensus to indicate which assay type is the most suitable for measuring circulating FGF-23. If all circulating FGF-23 is intact and biologically stable, concentrations detected by the intact and C-terminal assays should be comparable [10]. However, there is a paucity of data confirming this and so caution should be used when comparing studies using the different methods.

Several studies have assessed the performance of commercially available enzyme-linked immunosorbent assays (ELISAs) for FGF-23. Heijboer and co-workers evaluated the performance of one cFGF-23 assay (Immutopics, USA) and two iFGF-23 assays (Immutopics and Kainos Laboratories Inc., Japan) using samples from healthy volunteers and patients with expected high levels of FGF-23 [11]. Intra- and inter-assay variations were assessed in approximately 100 samples with low, normal and high FGF-23 concentrations providing <20% CV for the cFGF-23 Immutopics and iFGF-23 Kainos assays. A high intra-assay variation (22–61%) was observed for the Immutopic intact assay which may be due to lot-to-lot variation [11]. A potential difficulty observed with the Kainos intact assay is poor assay performance when using an automated plate washer, as directed in their protocol. Heijboer and co-workers found acceptable results were only obtained when the wells were washed manually, which made this method impractical for measurement of large numbers of samples [11]. However, a later version of the Kainos assay protocol (from October 2010 onwards) includes an improved wash instruction. Using plasma samples from patients with renal impairment, Devaraj and co-workers also found good inter- and intra assay precision for cFGF-23 assay (CV between 4–10.5%), however, the CVs for iFGF-23 Immutopic assay were found to be poor (6–37.5%) [12]. Poor analytical agreement exists between the commercially available FGF-23 assays, due principally to the lack of a reference method. The performance of four commercially available methods [iFGF-23 assays from Immutopics, Kainos and Millipore (USA) and a cFGF-23 assay from Immutopics] were recently compared using plasma from 31 healthy adults and 36 patients undergoing hemodialysis [13]. A broad range of FGF-23 values were obtained: whereas the patient ranges fell between 154–2561 pg/mL and 447–2063 pg/mL for iFGF-23 and cFGF-23 assay respectively, the levels for healthy adults ranged from 9.9–62 pg/mL for the two assay types. Poor analytical agreement was observed between the assays particularly in the patient group. No agreement of test results was found between the iFGF-23 and cFGF-23 assays and this was more evident at physiological concentrations than in the haemodialysis group [13]. The lack of analytical agreement between these commercially available FGF-23 methods emphasizes that they cannot be used interchangeably and that a comparison of findings from different assays requires careful interpretation. The above evaluation study was performed with plasma samples [13]. A further consideration is some assays are restricted to the sample type that can be used. The iFGF-23 Kainos assay is suitable for both plasma and serum; however, the cFGF-23 Immutopics assay is established only for plasma [10, 13] providing lower or undetectable results in serum [10, 12]. Further method comparisons, ideally on larger numbers of samples and in different patient groups, would provide a valuable insight in this area and help identify which assay type is the most suitable for measuring circulating FGF-23. Nevertheless, studies measuring either intact or C-terminal FGF-23 have reported associations with mortality risk and decline in renal function [4, 14]. Stability of FGF-23: implication for large scale epidemiological studies
Limited evidence exists for the short-term stability of FGF-23 in collected blood samples (6 hours or less) and no information is available for its long-term stability in stored samples. Smith and co-workers investigated the short-term pre-analytical stability of FGF-23, measured using iFGF-23 and cFGF-23 ELISAs from Immutopics, by performing a number of timed experiments with blood taken from 15 patients with mild CKD [6]. The effect of aprotinin, a serine protease inhibitor, and a commercially available protease inhibitor cocktail to preserve FGF-23 after blood collection was also investigated [6]. In the absence of any preservative or inhibitor, iFGF-23 degraded by approximately 40% within 2 hours of collection even when the blood samples were separated into plasma. Conversely, 2 hours after blood collection the FGF-23 concentrations had increased by approximately 35% using the cFGF-23 assay. However, with the addition of the protease inhibitor cocktail the stability of both iFGF-23 and cFGF-23 in the samples extended up to 4 hours (less than 10% change). Based on this evidence, it appears that FGF-23 cannot be measured reliably in blood samples collected without the use of any preservative or inhibitors. This could be a serious limitation for large-scale epidemiological studies, particularly if samples have already been collected and are in storage and for blood collection methods that need to be simple in order to be cost-effective and feasible.

Preliminary findings from our laboratory using samples from 54 CKD patients suggest that FGF-23 (measured by both the iFGF-23 Kainos ELISA and the cFGF-23 Immutopics ELISA) remains stable in whole blood stored for up to 96 hours without the use of a preservative [15]. The apparent lack of agreement with the results from Smith et al. may be explained by differences in the methods of sample collection. Smith et al. used a single K2-EDTA blood tube for each participant which was re-sampled at each time-point whereas we collected separate blood tubes corresponding to each time-point.
Large-scale epidemiological studies often involve the long-term frozen storage of samples prior to biomarker analyses, particularly nested case-control designed studies where it may take several years for sufficient incident cases to materialize. Limited information is available to help understand the impact of long-term frozen storage on the stability of many biomarkers, including FGF-23. Studies investigating the stability of biomarkers in different sample types stored at various temperatures (for example, −40 °C, −80 °C and in liquid nitrogen vapour) will have immense value, particularly in support of long-term blood based prospective studies and biobanks.

Summary
FGF-23 is a key regulator of phosphate homeostasis and has emerged as an important biomarker in patients with CKD. Despite an increasing amount of literature, there are still unanswered questions related to FGF-23 sample stability and the availability of robust reliable methods for measuring FGF-23. Further studies into these areas will improve the quality of clinical research into the use of FGF-23 as a potential early biomarker in CKD.

References
1. ADHR Consortium. Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF23. Nat Genet. 2000; 26: 345–348.
2. Wolf M. Forging forward with 10 burning questions on FGF23 in kidney disease. J Am Soc Nephrol. 2010; 21: 1427–1435.
3. Quarles LD. Role of FGF23 in vitamin D and phosphate metabolism: implications in chronic kidney disease. Exp Cell Res. 2012; 318: 1040–1048.
4. Isakova T, Xie H, Yang W, et al. Fibroblast growth factor 23 and risks of mortality and end-stage renal disease in patients with chronic kidney disease. JAMA 2011; 305: 2432–2439.
5. Russo D, Battaglia Y. Clinical significance of FGF-23 in patients with CKD. Int J Nephrol. 2011; 2011: 364890.
6. Smith ER, Ford ML, Tomlinson LA, Weaving G, Rocks BF, Rajkumar C, Holt SG. Instability of fibroblast growth factor-23 (FGF-23): implications for clinical studies. Clin Chim Acta 2011; 412: 1008–1011.
7. London GM, Guerin AP, Marchais SJ, Metivier F, Pannier B, Adda H. Arterial media calcification in end-stage renal disease: impact on all-cause and cardiovascular mortality. Nephrol Dial Transplant. 2003; 18: 1731–1740.
8. Jean G, Terrat JC, Vanel T, Hurot JM, Lorriaux C, Mayor B, Chazot C. High levels of serum fibroblast growth factor (FGF)-23 are associated with increased mortality in long haemodialysis patients. Nephrol Dial Transplant. 2009; 24: 2792–2796.
9. Titan SM, Zatz R, Graciolli FG, dos Reis LM, Barros RT, Jorgetti V, Moyses RM. FGF-23 as a predictor of renal outcome in diabetic nephropathy. Clin J Am Soc Nephrol. 2011; 6: 241–247.
10. Shimada T, Urakawa I, Isakova T, Yamazaki Y, Epstein M, Wesseling-Perry K, Wolf M, Salusky IB, Jüppner H. Circulating fibroblast growth factor 23 in patients with end-stage renal disease treated by peritoneal dialysis is intact and biologically active. J Clin Endocrinol Metab. 2010; 95: 578–585.
11. Heijboer AC, Levitus M, Vervloet MG, Lips P, ter Wee, PM, Dijstelbloem HM, Blankenstein MA. Determination of fibroblast growth factor 23. Ann Clin Biochem. 2009; 46: 338–340.
12. Devaraj S, Duncan-Staley C, Jialal I. Evaluation of a method for fibroblast growth factor-23: a novel biomarker of adverse outcomes in patients with renal disease. Met Syndr Relat Disord. 2010; 8: 477–482.
13. Smith ER, McMahon LP, Holt SG. Method-specific differences in plasma fibroblast growth factor 23 measurement using four commercial ELISAs. Clin Chem Lab Med. 2013; 51: 1971–1981.
14. Fliser D, Kollerits B, Neyer U, Ankerst DP, Lhotta K, Lingenhel A, Ritz E, Kronenberg F, Kuen E, Konig P, Kraatz G, Mann JF, Muller GA, Kohler H, Riegler P. Fibroblast growth factor 23 (FGF23) predicts progression of chronic kidney disease: the Mild to Moderate Kidney Disease (MMKD) Study. J Am Soc Nephrol. 2007; 18: 2600–2608.
15. Illingworth N, Edmans M, Clark S, Kumar A, Sutherland S, Herrington W, Hill M. Investigation of FGF-23 sample & assay suitability for large scale epidemiological studies. Ann Clin Biochem. 2013; 50: 73–74

The authors
Aishwarya Kuma^r1* PhD; Will Herrington² MBBS, MRCP; Sarah Clark¹ PhD; and Michael Hill¹ PhD
¹Clinical Trial Service Unit and Epidemiological Studies Unit (CTSU), University of Oxford, Oxford, UK
²Oxford Kidney Unit, Oxford University Hospitals, Oxford, UK

*Corresponding author
E-mail: Aishwarya.Kumar@ctsu.ox.ac.uk

Facilitating kidney function testing through the use of reagents applicable to a variety of automated systems

, 26 August 2020/in Featured Articles /by 3wmedia

The kidneys play an important role in homeostasis, they regulate the amount of water and salts present in the body by filtering blood through the nephrons. Waste products are filtered out and eliminated from the body in the urine, which is made up of the excess water, salts and waste products.

When the kidneys are not functioning efficiently, waste products and fluids begin to accumulate instead of being excreted which can cause serious health problems. Furthermore, kidney disorders can often develop and advance over a period of time without showing any signs; alternatively, symptoms are not recognized as being associated with kidney problems. Kidney function testing is therefore relevant for diagnosing and monitoring disease and assists in the development of appropriate treatment plans. Laboratory automation facilitates the efficiency and productivity of clinical laboratories. The determination of parameters related to kidney function by using tests incorporating reagents applicable to a variety of automated analysers facilitates clinical effectiveness and patient outcomes when managed by qualified laboratory professionals.

Kidney function assessment
Many conditions can affect the ability of the kidneys to carry out their vital functions. Some conditions can lead to a rapid (acute) decline in kidney function; other conditions lead to a gradual (chronic) decline. A number of clinical laboratory tests in blood and urine can be used to assess renal function. The unit measure of kidney function is the glomerular filtration rate (GFR), which can be defined as the volume of plasma cleared of an ideal substance –freely filtered at the glomerulus and neither secreted nor reabsorbed by the renal tubules- per unit of time. The normal range is 80-120 ml/min. Measuring this rate is a laborious process. Creatinine is the closest to an ideal endogenous substance for measuring GFR.[1,2] Creatinine is derived from creatine and creatine phosphate in muscle tissue and is defined as a nitrogenous waste product. Creatinine is not reutilized but is excreted from the body in the urine via the kidney. As a consequence of the way in which creatinine is excreted by the kidney, its measurement is used almost exclusively in the assessment of kidney function.

Urea, a byproduct of protein metabolism, is produced in the liver and then is filtered from the blood and excreted in the urine by the kidneys. The blood urea nitrogen test (BUN) measures the amount of nitrogen contained in the urea, high levels can indicate kidney dysfunction. As these levels are also affected by protein intake and liver function, this test is usually done together with a blood creatinine test.
Cystatin C is a small cysteine proteinase inhibitor that is steadily produced by all nucleated cells. The small molecular weight of cystatin C allows it to be freely filtered by the glomerular membrane and therefore cystatin C levels in the blood are indicative of a normal or impaired GFR. Levels of cystatin C in serum/plasma are almost entirely dependent on GFR.[3]

Other tests for the measurement of other parameters regulated in part by the kidneys can also be useful for the evaluation of kidney function; these tests include electrolytes (sodium, potassium, chloride, bicarbonate), protein, uric acid and glucose:

Electrolyte balance is essential for the normal function of cells and organs. Renal problems can be the cause of increased (hypernatremia) or decreased (hyponatremia) sodium concentration in blood. Potassium is normally excreted by the kidneys, elevated concentrations of potassium (hyperkalemia) can be an indicator of reduced renal function and potassium testing is also useful in the monitoring of patients receiving dialysis treatment. Increased (hyperchloremia) or decreased (hypochloremia) chloride concentrations can be a consequence of certain kidney diseases. The bicarbonate test is performed to monitor the acidity of the blood and other body fluids. Disruptions in the normal level of bicarbonate may be seen in kidney diseases.

The determination of total urinary protein is valuable in the evaluation of kidney function as healthy kidneys filter all the proteins and then return them to the bloodstream whereas damaged kidneys have diminished filtering ability and protein is therefore found in the urine.

Albumin is one of the major plasma proteins and in normal circumstances, is too large to cross the renal glomerular basement membrane. When damage occurs, the permeability of this membrane can change and albumin is then able to enter the urine. Albumin excretion rate of 20-200 µg/min is defined as microalbuminuria.[4]

Uric acid is a waste product excreted by the kidneys. Overproduction or reduced excretion of uric acid causes elevated concentrations of this molecule in the blood (hyperuricemia) or the urine (uricosuria). Increased concentrations of uric acid can form crystals in the joints (leading to joint inflammation and pain characteristic of gout) or in the kidneys (kidney stones that can cause kidney damage).

Abnormalities in glucose metabolism have been reported in patients with renal failure.[5]

Application of kidney function tests to automated systems
In clinical settings the application of tests for the determination of parameters related to kidney function to automated systems, facilitates clinical effectiveness and productivity. There are currently tests available for the determination of creatinine, BUN, cystatin C, electrolytes, protein, uric acid and glucose among others. If a variety of these tests could be applied to one system, the result output for each system would increase, which would maximize efficiency. The use of tests incorporating reagents applicable to a variety of automated analysers is beneficial as it increases the testing capacity of one system. This is further enhanced by the analyser’s capability to employ different methodologies with different reagents. The combination of automation and the use of stable, high performance reagents, lead to optimal analytical performance, extensive measuring ranges to ensure detection of abnormal values and reduced interference to produce more accurate results. For instance, a study using a creatinine test reported no interference with bilirubin and metamizol.[6]
The application of other kidney function related tests to studies in patients with nephrotic syndrome, chronic liver diseases and diabetes have also been reported.[7-8]
The automation of laboratory testing still requires qualified laboratory professionals for the evaluation of the results but reduces errors, staffing concerns and safety issues. This facilitates the diagnosis and the monitoring of kidney function, which is of great importance in clinical practice and in research.

Conclusion
The kidneys are the body’s natural filtration system and perform many vital functions. Kidney function tests is a collective term for a variety of individual tests and procedures for the evaluation of how well kidneys are functioning. The determination of parameters related to kidney function (i.e.creatinine, BUN, cystatin C, electrolytes, protein, uric acid, glucose) by using tests incorporating reagents applicable to a variety of automated analysers, increases the testing capacity of the systems and facilitates clinical effectiveness and patient outcomes when managed by qualified laboratory professionals.

References
1. Berger A. Renal function – and how to assess it. BMJ. 2000; 321: 1444.
2. Traynor J, Mactier R, Geddes CC, Fox JG. How to measure renal function in clinical practice. BMJ. 2006; 333 (7571): 733-737.
3. Laterza OF, Price CP, Scott MG. Cystatin C: an improved estimator of glomerular filtration rate? Clin. Chem. 2002; 48(5): 699-707.
4. Kirby M. Screening for microalbuminuria. The British Journal of Diabetes and Vascular Disease. 2002; 2(2): 106-109.
5. Sechi LA, Catena C, Zingaro L., Melis A, De Marchi S. Abnormalities of glucose metabolism in patients with early renal failure. Diabetes. 2002; 51: 1226-1232.
6. Harmonien AP. Bilirubin and metamizol do not interfere with the Randox enzymatic creatinine test. An evaluation of a new enzymatic creatinine determination method. Eur. J. Clin. Chem. Clin. Biochem. 1996; 34(12): 975-976.
7. Mula-Abed W-AS and Hanna BE. Measurement of serum fructosamine as an index of glycated protein in patients with nephrotic syndrome and chronic liver diseases. Bahrain Medical Bulletin 2001; 23(4).
8. Hirnerova E, Krahulec B, Strbova L, Stecova A, Dekret J, Hajovska A, Ch A Dukat A. Effect of vitamin E supplementation on microalbuminuria, lipid peroxidation and blood prostaglandins in diabetic patients. Bratisl. Lek. Listy 2004; 105(12): 408-413.

Author
María Luz Rodríguez
Randox Laboratories Limited,
55 Diamond Road, Crumlin,
County Antrim, N. Ireland, BT29 4QY,
United Kingdom