The evolution of rapid diagnostic testing in the management of influenza

The use of point-of care (POC) testing, office/bedside diagnostic measurements with rapid and simple-to-use testing procedures, has greatly increased since the introduction of rapid tests. The popularity of POC testing is based on both medical considerations (reduction in both diagnostic time and unneeded and/or inappropriate medications) and cost savings to the clinic or hospital. A consideration of how such tests are used in a specific clinical setting for diagnosis and management of influenza provides an opportunity to consider the advantages and limitations of current POC testing and to review advances that may increase its utility in the future.

There are currently more than 10 rapid influenza diagnostic tests (RIDTs) that have been approved by the Food and Drug Administration (FDA). These tests are typically immunochromatographic assays that detect specific influenza viral nucleoprotein antigens in respiratory specimens and provide qualitative results (positive or negative) in 10 to 30 minutes. Some RIDTs are approved for use in the POC setting. Despite their outward similarity, RIDTs differ in both the information that they provide and in their performance characteristics. Most RIDTs detect both influenza A and B viruses and can distinguish between them. But some RIDTs identify only A or B viruses, and some RIDTs that identify both A and B cannot distinguish between them.¹

Recently, two companies have received CLIA-waivers for new assays that include an objective, reader-based interpretation of results (as opposed to a subjective, visual read for all prior RIDTs). One of the assays uses reverse transcriptase polymerase chain reaction (RT-PCR), which is a much more sensitive comparator than culture as a reference for determining positive percent agreement (sensitivity) and negative percent agreement (specificity).

The ability of RIDTs to detect common subtypes of seasonal influenza such as swine flu (influenza A, subtype H3N2v) is always a question. In a recent preliminary assessment by the Centers for Disease Control and Prevention (CDC), the ability of seven widely used, FDA-approved RIDTs to detect H3N2v samples was evaluated.² Five samples of each virus were diluted tenfold with physiological saline and tested in duplicate with each of the RIDTs following procedures described in the test kit labeling. Four of the seven RIDTs tested detected all of the H3N2v viruses. The other three detected from one to five of the seven virus samples.² These results indicate that only some RIDTs are capable of detecting H3N2v.

RIDTs vary widely in terms of accuracy. A recent meta-analysis that examined the accuracy of RIDTs based on the results of 159 published studies of the sensitivity and specificity of 26 different RIDTs found that while the pooled sensitivity of the tests was 62.3%, the sensitivities in individual published studies included in the meta-analysis ranged from 4.4% to 100%. Pooled specificity of all of the studies was better—98.2%. Only studies that compared the diagnostic results of RIDTs with results from either viral culture or from the more sensitive RT-PCR assay as references were included in the meta-analysis.³

Several factors can have major effects on the positive and negative predictive values of RIDTs. Collection of samples for testing should follow the guidance of the package insert in the test kit, since results can vary with the type of sample (e.g., throat swab versus nasal swab). The prevalence of influenza in the community at the time of sampling can also affect the predictive value of any assay, including RIDTs. Although the false-positive rate for results does not change, the predictive value is lower when influenza prevalence is low, generally at the beginning or end of the influenza season. Alternatively, a positive test during times of high disease prevalence indicates that infection with influenza virus is likely.¹

The time of sample collection relative to the onset of symptoms can also influence the outcome of testing. An infected person is contagious for up to seven days after the onset of illness and sheds virus from one day prior to the onset of symptoms until the resolution of fever. A sample collected from an infected person within 48 to 72 hours of disease onset is more likely to yield positive RITD results than a sample collected later in the course of the disease.⁴

For clinicians, the major advantage of RIDTs is their ability to provide results in a clinically relevant time frame. For example, the time from introduction of the sample to result can be as short as 10 minutes. Rapid diagnosis of influenza permits treatment with antiviral agents that can be effectively used only within a 48-hour time interval after the onset of disease. The major disadvantage of RIDTs historically has been suboptimal performance of many of the tests, particularly the rate of false negative results (sensitivity) when influenza activity is high. The CDC recommends that negative results of an RIDT be followed up with a confirming test such as culture or PCR. Physician judgment as to the presence of the signs and symptoms of influenza, the level of influenza activity in the community at the time of testing, and prior exposure of the infected person to pigs or birds should also be considered when making the decision to treat the patient for influenza. Additional influenza testing should be considered if the patient tests positive with RIDT at a time when the community prevalence of influenza is low.⁴

RIDTs can be particularly useful for identifying influenza virus infection as a cause of respiratory outbreaks in institutions such as nursing homes, chronic care facilities, and hospitals. In this situation, it is important to determine whether influenza A or B is the cause of the infection so that appropriate preventive intervention can be employed to limit the outbreak. Positive RIDT results in persons with suspected influenza can serve to validate decisions to implement prevention and control protocols for influenza outbreaks. Using an RIDT to test respiratory specimens from several persons with suspected influenza will increase the likelihood of identifying influenza virus infection as the cause of the outbreak. Regardless of the outcome of testing, hospitalized patients with suspected influenza should receive immediate antiviral treatment. In the case of a suspected institutional outbreak, public health authorities should be notified and respiratory specimens collected from symptomatic persons and sent to a public health laboratory for follow-up influenza testing.⁴

Advances in RIDT technology provide an opportunity for increases in the sensitivity of testing. For example, while older RIDTs rely on subjective visual evaluation of test strips to determine the outcome of a test, more advanced RIDTs use advanced technology and readers that provide an objective, digital determination of the presence of viral antigens. The digital reader eliminates the need for visual reading and subjective interpretation of the results of the test. Newer technology also employs assay algorithms that can assist in distinguishing antigen-specific antibody binding from non-specific binding.⁵

Traditionally, the faster turnaround time of RIDTs has tended to offset the reduced sensitivity of many tests compared with cell culture or RT-PCR, which can take days or hours, respectively, to produce a diagnosis and typically require specialized equipment and trained personnel. In recognition of their usefulness for detection and management of influenza, RIDTs are included in the guidelines for influenza management of the Infectious Diseases Society of America, the CDC, and the World Health Organization.⁴

These recent advances may provide laboratorians and clinicians with a more definitive test result and ultimately lead to a more prominent role for RIDTs.