Sound diagnostics: rapid point-of-care nucleic-acid based tests for sexually transmitted infections

One of the major avenues for addressing the rising impact of sexually transmitted infections lies with rapid, early diagnosis to break the cycle of transmission. Here we discuss the potential of a new technology, using the mechanical energy of sound waves, to drive integrated point-of-care diagnostics.

by Dr Julien Reboud, Gaolian Xu and Prof. Jonathan M. Cooper

Point-of-care diagnostics for sexually transmitted infections
Infectious diseases have a huge impact on both health and morbidity – causing more than half of the deaths in low-resource countries. To reduce the impact of these diseases, it is now accepted that early diagnosis is needed in order to break the cycle of infection and transmission. The development of rapid, high performance molecular diagnostic technologies, such as those involved in nucleic acid testing (NAT), has the potential to provide a much-needed step-change in treatment, through the early diagnosis of infection. Importantly, NATs can also be used to identify resistant strains of bacteria, an important step-change in the fight against the evolution of antimicrobial resistance (AMR).

One group of diseases that continues to increase in all areas of the world are the sexually transmitted infections (STIs). For example, chlamydia (caused by Chlamydia trachomatis) and gonorrhoea (caused by Neisseria gonorrhoeae) remain highly prevalent throughout the world. The WHO/CDC estimate chlamydia to affect 11m in Europe/Central Asia and 5.2m in the US per year; with gonorrheoa affecting 1.1m in Western Europe and >0.7m in the US per annum.

Sexual health clinicians have rated point-of-care (POC) testing as their top priority with their key concern being ‘in-clinic’ latency. Current testing protocols using NATs require an amplification process such as polymerase chain reaction (PCR) or isothermal amplification (e.g. loop-mediated isothermal amplification (LAMP)). When implemented in a laboratory or clinic, the workstream often requires sending samples to an external laboratory, a process that takes several hours. This results in the patient leaving the clinic. Patients then have to be recalled to the clinic for treatment, during which time they remain infectious for others and at risk of developing complications from the infection. Some never return, and remain untreated and a risk to others. The most vulnerable patients from high-risk groups such as the very young or men who have sex with men are less likely to engage with services. About 10% of all those diagnosed in the National Chlamydia Screening Programme in England in 2012 have never been treated. Those patients presenting to clinical services who report recent exposure to chlamydia or gonorrhoea may be treated with antibiotics pending their lab results, even though around half will turn out not to be infected. Treatment for gonorrhoea now involves parenteral third-generation cephalosporins combined with an oral antibiotic, and there is evidence of increasing drug resistance. Good antibiotic stewardship seeks to limit unnecessary exposure of the population to these agents.

POC testing is a paradigm closely associated with self-diagnosis. Such near-patient devices are easy to use (by untrained people) and are rapid. Other characteristics include the integration of processing steps from sample to answer at a low cost [1]. POC testing of STIs would not only be relevant in developed healthcare systems, but also in the home (bathroom testing) as well as in resource-limited countries (where testing would often be delivered by a healthcare worker within a community) [2]. In all cases, the ability to ‘multiplex’ (testing multiple possible infections) and provide decision support around treatment are desirable. As stated, much evidence already exists that such a test would be desired by both by clinicians [3] and patients [4]. POC testing for chlamydia for example is also likely to be cost-effective. A mathematical model using costings from one of the few commercially available POC tests (Cepheid Xpert CT/NG) was shown to reduce testing costs by up to £16 and save 10 minutes of a healthcare professional’s time per patient [5].

Although there has been significant development in technological research for highly sensitive sensors, along with integrated microfluidic devices, the widespread adoption of POC tests has been limited by appropriately sensitive performance in real patient samples (blood, saliva, urine or feces, for example). Notwithstanding this, the relevance of decentralizing testing has been evidenced in Australia, for example, where a historical systematic review of interventions to prevent HIV and STIs in young people found that testing increased if a non-clinical, non-primary care healthcare setting was used [6]. This data confirms what many clinicians are aware of, that in the specific case of sexual health, there is a reticence for individuals to engage formally with healthcare systems.

Acoustic technology for lab-on-a-chip POC diagnostics
Many proposed lab-on-a-chip devices currently rely on a variety of different mechanisms for preparing the sample prior to sensing, such as external pumps and heaters, leading to expensive and complex systems. In addition, microfluidic systems are often constrained by both difficulties associated with the chip interconnection to other instruments, and by difficulties that arise as the sample is moved through the chip (not the least of these being blockages). One outcome is that such diagnostic chips tend to be complex – a fact that increases the cost of the manufacture of the chip and ultimately the cost of the test. We have developed a new technology based on surface acoustic waves to integrate sample manipulation onto low cost disposable devices to enable the multiplexed detection of chlamydia and gonorrhoea, using isothermal amplification [7].

Acoustic waves contain a mechanical energy that can be used to manipulate fluids. A range of ultrasonic transducers have already been developed, including those using both bulk acoustic waves (BAWs) and surface acoustic wave (SAW) devices [8]. Here we use a widespread configuration where a high frequency electric field is applied to a piezoelectric chip to create an ultrasonic wave, which propagates into the sample. We have now demonstrated a new proprietary technology using the interaction of SAW with fluids and phononic metamaterials [9] that has enabled us to create a tool-box’ of different diagnostic/medical instrumentation functions (including sample processing, cell separation [10], cell lysis [11], PCR [12] and nebulization [13]). Just as in electronics, where discrete components are combined to create a circuit, so we have begun to use different combinations of phononic lattices to create fluidic microcircuits, each of which provides a unique diagnostic function. The approach removes the need for any off-device processing, making sample processing a seamless, simple and fully automated process. Unlike conventional microfluidics, where the sample moves through the chip, our technology simply relies upon controlling the excitation frequency of the acoustic fields within a stationary droplet.

We have recently demonstrated the implementation of isothermal amplification (through LAMP) on our acoustic platform [7], enabling the multiplexed detection of both chlamydia and gonorrhoea on a single disposable device, down to a sensitivity of 10 copies. Uniquely, the acoustic platform results in faster detection, through accelerated mass transfer, which is of paramount importance for a POC platform. We believe that the ease of implementation of both SAW technology and LAMP will have the potential to significantly impact upon near-patient diagnostics.

The authors are grateful for the help of Dr Rory Gunson and Andrew Winters (NHS) for their input into the development of the STI technology.

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The authors
Julien Reboud* PhD, Gaolian Xu MSc, Jonathan M. Cooper PhD
Division of Biomedical Engineering, School of Engineering, University of Glasgow, Glasgow, UK

*Corresponding author