The pending wave of point of care molecular testing: outlook and initial observations
When we consider molecular diagnostic (MDx) lab testing, it is in most cases in the context of a centralized “core lab.” There are a number of purely practical reasons for this; for example, a centralized lab can be equipped to perform a wide range of tests, meaning a single delivered sample can be readily assayed for multiple analytes as needed. It can be staffed by a group of people with focused training on the performance and interpretation of the specific test menu offered. Frequently, it can also provide opportunities for economies of scale in equipment and reagents, when compared to multiple disperse locations with mirrored test capabilities. Particularly in the molecular context, where template or amplicon contamination is an ever-present risk for false positive results, infrastructural aspects designed to mitigate these risks (such as room sanitation by UV irradiation, airflow control, or workflow directionality and compartmentalization within rooms and/or process dedicated biosafety cabinets) are expensive, impractical, or both to implement in non-dedicated lab space.
Regardless of these practical considerations, a frequently expressed desire in clinical testing is to move individual tests out of the core lab and closer to the patient. The motivation for this is obvious: faster diagnosis allows for faster selection of appropriate medical intervention(s) for the patient. This is associated not just with better clinical outcomes, but also lower costs to the entire healthcare system. In idealized situations, accurate and rapid testing could be done right in the physician’s examination room with clearly interpretable results available before the patient leaves. These and similar scenarios are generally referred to as “point-of-care” or POC testing, and the expansion of molecular methods into the POC setting has both significant challenges and promises.
High, moderate, waived
Perhaps the most significant hurdle to moving MDx processes into POC settings is what’s lumped under the term “complexity.” Under U.S. CLIA regulations (and similar in other regulatory jurisdictions), tests are classified as high, moderate, or waived (i.e., low) complexity. As the names suggest, high or moderate complexity tests are ones that require significant specialized infrastructure to perform accurately, and often specialized training in their performance or result interpretation. Waived tests, on the other hand, are (and I quote from CLIA) “simple tests with a low risk for an incorrect result. They include certain tests listed in the CLIA regulations, tests cleared by the Food and Drug Administration (FDA) for home use, and tests approved for waiver by the FDA using the CLIA criteria.”
Low risk for an incorrect result is a difficult goal to achieve, as it requires both that the test process be very simple and reliable and that the interpretation be simple and obvious. Common examples of waived tests include home pregnancy tests and many of the rapid “immunospot” type tests such as those seen for the detection of common viruses. By nature, waived tests lend themselves to POC application, although the reverse is not strictly true: that is, in order to be performed as a POC test, a test does not necessarily have to be in the waived category. I quote again from CLIA: “’Point-of-care testing’ is a phrase used to describe the location where testing is performed, such as at the bedside or near the site of patient care. While some point-of-care tests are approved for a CLIA waiver, advances in technology that enhance the rapidity of testing are allowing more complex, nonwaived testing to be performed at or near the site of patient care.”
Put another way, historically, waived and POC testing were nearly synonymous, and waived class assays remain the most straightforward ones to put into POC settings. As anyone with personal front-line experience with many of the older POC test systems (particularly the immunospot-style pathogen tests) will know, the speed and convenience of these tests often came at a cost in sensitivity, specificity, or both when contrasted to higher complexity, slower tests performed by a core lab. That doesn’t mean they weren’t useful; for example, POC tests with only moderate sensitivity but high specificity can be a highly cost-effective first-line screening tool, with detected positives being accepted and allowing for immediate assignment of appropriate medical action. Less total samples (only those scoring negative in the POC test, or otherwise dictated for special considerations) get passed on to the slower, more accurate, but generally more costly centralized lab test of moderate or high complexity.
MDx and POC
Imagine, now, if we could develop POC tests based on molecular methods, with the inherently exquisite sensitivity and specificity of molecular diagnostics. The challenges in achieving this, particularly if the intent is to achieve waived status, are not trivial, as any such test system must be built in such a manner as to be highly resistant to contamination, while yielding readily interpretable results all in the hands of what would normally be referred to as “untrained users” without in-depth training on the particular assay. The potential reward, however, is significant, as such a test would be expected to enjoy the benefits common to POC tests, while providing enhanced sensitivity and/or specificity.
That reward has been incentive behind many avenues of active research and development—one of which led to the first commercial release of a molecular test (for influenza A and B), with CLIA-waived status, in January 2015. This was followed only months later by CLIA-waived status approval for a molecular assay for Group A Streptococcus. While these were all on a single platform, these approvals really mark a critical milestone for all organizations looking to develop POC molecular testing. That milestone is the acceptance from a regulatory standpoint that molecular testing can be made in a format other than high-complexity and thus forcibly relegated to a centralized laboratory. Other systems will surely follow down this pathway, and the POC testing environment is likely to undergo significant changes in the near future as older, less sensitive and specific test modalities in these settings begin to acquire the benefits of MDx.
How do the sensitivities and specificities of these first waived molecular tests compare, both with older immunospot POC tests and with core lab mainstream MDx systems? That question is difficult to answer in anything more than a very general sense, not only because of the wide performance range of available immunospot or similar non-molecular POC tests and the diversity of “central lab” molecular platforms and tests, but also because there are only a few of these waived molecular tests to compare against. With those caveats in mind, however, the results appear very promising. The molecular waived flu A/flu B test referred to above has stated PPA (positive percent agreement) of 95 percent and NPA (negative percent agreement) of 96 percent on influenza A, and PPA 82.5 percent / NPA 98.4 percent for influenza B (according to the package insert). While these values clearly show room for improvement, they are appreciably better than this author’s experience with non-molecular POC tests for these same targets and only slightly below what could be expected with some current core-lab molecular methods.
Further considerations
Is this the beginning of the end for the molecular core lab? Far from it. One important reason for this is that these POC tests, and likely any that would be imagined in the near future, remain tests for at most a few specific analytes. An important strength of the core lab, and one that remains unchallenged by these advances, is the uncoupling of nucleic acid extraction from sequence-based testing; that is, once a sample is received and nucleic acids extracted, the extract can be queried by a wide range of tests either all at once, or in a descending “flow scheme” as the results of first sequential tests turn up negative. By contrast, the sample dedicated to POC type tests is lost to further testing once allocated.
Rather than seeing emerging POC molecular tests as a challenge, the agile and forward-thinking core molecular lab will actively embrace and support the use of these tests as a means to move screening for many common targets out to the physician’s offices, ERs, and hospital receiving rooms. This will have the potential to free up the more diversified capabilities and directed methodological and interpretive training of the core lab staff to focus on more challenging and less commonplace samples. If this is coupled with driving core lab capabilities further into emerging technologies such as next generation sequencing, the result will be a new lab/POC balance in molecular methods.
At least one additional factor to bear in mind in a setting where molecular techniques are brought into POC use for infectious diseases is that the front-line test users should be aware—either through product literature, training, or consultative communication with specialized core labs—that a molecular positive is not synonymous with a viable infectious organism. Without this, front-line users may not appreciate the concept of persistence of detectable organism nucleic acids past organism viability. Regardless of this and other similar challenges to be addressed through education, use of molecular techniques in POC settings can only be expected to increase and to improve patient care.
For interested readers, the CLIA quotes referenced may be found at:
https://wwwn.cdc.gov/clia/Resources/TestComplexities.aspx
