An early and widely cited clinical study on the use of calprotectin stated it to be stable in feces for seven days. However, it has subsequently been shown that in fecal samples left at room temperature, the mean change in baseline calprotectin is about 30% although it is stable at 2–8 °C for at least 10 days and at −20 °C for at least a year and can withstand at least four freeze− thaw cycles. Once extracted into buffer calprotectin may still be subject to degradation at room temperature but this has been found to depend upon the method-specific extraction buffer used. Delay in return of unrefrigerated samples to the laboratory, or their movement between laboratories, has potential to lead to degradation of calprotectin and a result which may be unrepresentative of a patient’s clinical status.
Use of an extraction device and rapid immunochromatographic assay together with a smartphone application now enables patients to measure their calprotectin at home and send results, via a secure portal, for review by their gastroenterologist; results have been shown to be comparable with those obtained by ELISA within a laboratory setting. Self-monitoring is welcomed by many patients and has great potential as a tool to facilitate virtual consultations, reducing the need for hospital visits.
A cut-off value of 50 μg/g is quoted by many manufacturers as the upper limit of the reference for fecal calprotectin in adults. However, data from a pilot External Quality Assurance scheme showed that the assay used had potential to affect patient classification around this cut-off. Calprotectin concentrations have been shown to be higher at the extremes of age; studies in healthy infants reporting median concentrations of about 250 μg/g with a ranges extending to over 2000 μg/g.
The choice of cut-off value for use in clinical pathways needs to balance sensitivity and specificity, reflecting that the test is not performed in isolation but in combination with assessment of clinical status. Minor and transient episodes of gastrointestinal inflammation can raise fecal calprotectin and so although a calprotectin level >150 μg/g suggests IBD, a single measurement of 50–150 μg/g may not. Depending upon the context, repeat analysis is likely to be appropriate. On repeat, ongoing elevation or a rising calprotectin level increases the likelihood of IBD. Because of the differences between manufacturers, laboratories should ensure that any cut-off values are optimized for the particular assay in use.
Distribution in feces
Although calprotectin has been shown to have a homogeneous distribution in feces, significant day-to-day variation has been observed, particularly in patients known to have IBD. It has also been observed that there is variation during the day and that concentrations of calprotectin increase with increasing time between bowel movements. Although it has been recommended that calprotectin should be measured from the first stool passed in the morning, more recent guidance suggests that when monitoring IBD it is better to measure more than once, with time of sampling being less important.
Non-steroidal anti-inflammatory drugs
Non-steroidal anti-inflammatory drugs, such as aspirin and ibuprofen are associated with a rise in fecal calprotectin. With short-term use, the reason for any elevation is likely to be the development of intestinal inflammation. Long-term use of these agents can cause a specific enteropathy, possibly the result of drug-induced suppression of prostaglandin production with dysregulation of blood flow and mucous production.
Measuring calprotectin concentrations is useful for the diagnosis and monitoring of IBD and has been proposed as a tool to rule out significant bowel disease.
However, there are specific pre-analytical and technical aspects that need to be taken into account when measuring calprotectin levels and interpreting results.