Hepatitis C Virus Diagnostics: The Road to Simplification

Watch a video presentation of this article Watch the interview with the author The ambitious goal of eliminating viral hepatitis as a public health problem by 2030 will require major efforts to prevent new infections, but perhaps more importantly, there will have to be a dramatic increase in diagnosing those already infected and linking them to care. Although the algorithm to make a diagnosis of hepatitis C virus (HCV) infection may seem simple to the experienced clinician, the need for multiple tests, some with slow turnaround times, is proving to be a major impediment to achieving elimination targets. The traditional approach to HCV diagnosis requires an initial antibody (Ab) test to document exposure, followed by a confirmation of ongoing viremia, usually using a polymerase chain reaction (PCR)–based assay to quantify the level of HCV RNA in the blood. Individuals who test Ab positive but HCV RNA negative have either spontaneously cleared the infection (usually within 6-12 months of initial infection) or been successfully treated. Other much less common possibilities are false-positive Ab tests or acute infection during which the HCV RNA can be transiently undetectable. Current Ab and HCV RNA assays have very high sensitivity and specificity, making false-positive and false-negative results rare occurrences.1 The real challenge of the traditional approach is the need for two blood tests (Fig. 1). Although this may not seem so difficult, when one walks through the journey for a patient, it quickly becomes apparent that there are many places to get lost along the way (Fig. 2). The individual must first see a provider who recognizes the risk for HCV infection and orders the initial Ab test. A trip to the phlebotomist to get the test done is then required, followed by a return clinic visit for the result. If positive, a second requisition for HCV RNA is sent, requiring a return trip to the laboratory and a return trip to the clinic for the result. If any one of the steps along the way to a confirmed diagnosis is missed, the individual may never know of the infection, leading to a missed opportunity for treatment and possibly a risk for inadvertent ongoing transmission. The many steps in the process disproportionally limit diagnosis in marginalized populations, many of which have high burdens of HCV infection.2 Challenges of the two-test approach for specific populations are highlighted in Table 1, ranging from the need for venipuncture to the requirement to see people on multiple occasions. The simplest solution to ensuring an HCV RNA test is done for all positive Ab tests is to reflex the testing. PCR can be performed on leftover serum after the Ab test; however, there are some issues to consider. RNA that is improperly stored may degrade. Fortunately, accurate quantification is rarely relevant with current therapy, and there is a low probability of a positive sample degrading sufficiently to become negative.4 To minimize degradation, if a single tube is collected that will be used for Ab and RNA testing, the collection times must be done as if for RNA, and the residual serum must be correctly stored while awaiting Ab results. This requires coordination if Ab and RNA testing are done in different laboratories or even different sections of the same laboratory. However, the ability of large commercial laboratories to operationalize this process demonstrates that it can be done effectively, with results showing near-universal completion of an RNA test on all Ab-positive results. Collecting two tubes at baseline is also possible but adds significant cost when one considers the amount of screening required worldwide. Another mechanism for enabling reflex testing is the use of dried blood spot (DBS) testing for HCV. The approach has been used in other fields, particularly human immunodeficiency virus (HIV), with great success.5 A fingerprick of whole blood is applied to a card with four or five individual filter paper spots (Fig. 3). After drying, the blood remains very stable on the DBS card, allowing for simple shipping, even using regular mail. Once in the laboratory, the serum is eluted off the card and can be run using the standard automated serum detection systems. The first spot is tested for Ab, and if positive, the next spot is tested for HCV RNA.6 Because of the lower quantity of blood, the HCV RNA loads from DBS are lower than in a serum tube from the same patient by ~2 log IU/mL.7 This is important if results are used for quantification, such as deciding whether to shorten therapy with sofosbuvir/ledipasvir, but is otherwise rarely clinically relevant because more than 95% of individuals with chronic infection have HCV RNA levels greater than 4 log IU/mL, making it extremely unlikely to go from positive in serum to negative on DBS.7 Another advantage is the lack of requirement for venipuncture, which is very helpful for people with a history of injection drug use and also enables testing to be done by individuals with minimal training, including peer workers. Also, other spots can be used for other serological tests (e.g., HIV, hepatitis B virus [HBV], and syphilis). Although potentially useful, particularly in rural and remote settings, DBS is not faster than standard serum testing and may affect workflow in the laboratory because of the requirement for an elution step. The ability to obtain an immediate test result avoids the need for return visits and tracking down individuals who do not return. Point-of-care (POC) tests have been developed for HCV Ab and, more recently, for HCV RNA. POC Ab tests are particularly useful in settings where follow-up may be challenging, such as screening drives or testing in emergency departments. The approved OraQuick test requires only a fingerprick and provides a result in 20 minutes, allowing for those testing positive to have an immediate follow-up test for HCV RNA. A version of the test using salivary fluid has been developed but has slightly lower sensitivity and was therefore not approved in North America.8 The option to avoid drawing blood at all may be a major advantage in certain settings and countries. Weighing the reduced sensitivity against increased testing uptake should be considered with considerations of the overall cost-effectiveness, which may favor this approach in many settings. Other POC Ab tests have also been developed. More recently, a rapid HCV RNA test has been Conformité Européene (CE)-marked in Europe. HCV RNA quantification can be done on the GeneXpert using a drop of whole blood or serum with a turnaround time of approximately 90 to 100 minutes. Individual cartridges for blood collection are placed into a module that does automated nucleic acid extraction and amplification.9 The test is not yet approved in North America but may be useful in many regions of the world that use the same platform for rapid diagnosis of other infections, such as tuberculosis and HIV, and thus already have the modules available in testing facilities. In very high prevalence settings (e.g., prisons, opioid substitution clinics), it may be reasonable to consider going directly to HCV RNA testing without initial Ab testing. The major advantage would be reducing the time for a confirmed diagnosis, which currently, even using rapid Ab and RNA testing, likely requires close to 2 hours. The success of direct-acting antivirals for HCV means that on-treatment monitoring is rarely required, which may allow for tests to move to qualitative (positive/negative) strategies with somewhat reduced sensitivity, that will hopefully improve the turnaround time for HCV RNA detection to make it truly POC.10 The purpose of HCV RNA testing is to confirm viremia. Detection of the core antigen (Ag), a part of the viral nucleocapsid, in the blood also confirms active infection. Assays for core Ag have been developed with good performance characteristics. The main limitation of core Ag testing is the slightly reduced sensitivity; that is, most assays have a threshold of detection equivalent to ~10,000 IU/mL of HCV RNA.11 More than 95% of individuals with chronic HCV have viral levels above this threshold, but there is a small but real risk for missing those with low-level viremia. The advantage of core Ag is primarily lower cost and possibly improved stability, given that it measures a protein rather than RNA. However, core Ag testing also requires a centralized laboratory and is not currently a rapid test. If POC core Ag test could be developed at low cost, it could potentially obviate the need for Ab testing at all, which would greatly simplify testing, particularly in resource-limited settings. There are pros and cons to the various approaches to HCV diagnosis that make certain strategies preferred in particular clinical settings. The cost of the test is not the only consideration. Changes in sensitivity and/or acceptability may markedly affect overall cost-effectiveness of testing strategies. Other points that must be considered are the availability and need for a centralized laboratory (e.g., DBS, core Ag versus POC test), feasibility of providing results and counseling (e.g., POC testing in emergency department), and acceptability of test/tester to affected populations (e.g., venipuncture challenges for people who inject drugs, use of peer testers with DBS). Optimal settings for the different strategies are highlighted in Table 2. With incredibly effective and very simplified HCV therapy, it is conceivable to imagine a scenario in which a person gives a drop of blood for rapid diagnosis of HCV followed by immediate treatment and a second drop of blood after therapy for confirmation of cure. Hopefully, the new technologies to realize this paradigm will be available in the near future. Until then, clinicians should work with their laboratory partners to streamline the diagnostic cascade using existing tools. Different solutions may work better in different clinical settings, but simplification, particularly the use of reflex RNA testing for Ab-positive samples, will be critical if we are going to reach the ambitious global elimination targets by 2030.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.