There’s an old joke about the airline pilot who comes on the intercom to tell the passengers that he has bad news and good news for them.
The bad news is, they are hopelessly lost. The good news is, they are making excellent time. . . .
Much the same could be said about the RIDT (Rapid Influenza Diagnostic Tests) that are commonly used in doctor’s offices and clinics.
They are often wrong . . . but on the plus side you can get the results in only 20 minutes . . .
The two main measures of the accuracy of a diagnostic test are sensitivity and specificity.
- Sensitivity is defined as the ability of a test to correctly identify individuals who have a given disease or condition.
- Specificity is defined as the ability of a test to exclude someone from having a disease or illness.
This rapid test is designed to show if someone tests positive for the Influenza A or B virus, but not the strain of flu.
The rapid tests vary in terms of sensitivity and specificity when compared with viral culture or RT-PCR. Product insert information and research publications indicate that:
- Sensitivities are approximately 50-70%
- Specificities are approximately 90-95%
While the limitations of the RIDTs have been known for some time, and the CDC recently cautioned doctors against relying on negative rapid tests to exclude a diagnosis of influenza, in today’s MMWR we get data that indicates that when dealing with the novel H1N1 virus, these rapid tests may be even less accurate than previously thought.
During May 2009, a few weeks after 2009 pandemic influenza A (H1N1) infection was first detected in the United States (1), outbreaks among students from two schools were detected in Greenwich, Connecticut. Staff members from Greenwich Hospital and the Connecticut Department of Public Health collected data on symptoms for 63 patients and submitted nasopharyngeal washings for testing using a rapid influenza diagnostic test (RIDT) for influenza A and B and real-time reverse transcription--polymerase chain reaction (rRT-PCR) assay, thereby affording an opportunity to assess the field performance of the RIDT.
A total of 49 patients had infections with pandemic influenza A (H1N1) confirmed by rRT-PCR. This report summarizes the findings from this performance assessment, which indicated that, compared with rRT-PCR, the sensitivity of the RIDT for detecting infection in patients with 2009 pandemic influenza A (H1N1) was 47%, and the specificity was 86%.
Sensitivity and specificity did not vary substantially by the presence or absence of CDC-defined influenza-like illness (ILI) or by time from symptom onset to specimen acquisition. In this group of patients, although positive RIDT results performed well in predicting confirmed infection with pandemic H1N1 virus (positive predictive value: 92%), negative tests did not accurately predict the absence of infection (negative predictive value: 32%).
These results affirm recent CDC recommendations against using negative RIDT results for management of patients with possible 2009 pandemic influenza A (H1N1) infection (2).
Essentially, fewer than half (47%) of those who were later confirmed to have the novel H1N1 virus were positive by the RIDT.
While a positive test was right 86% of the time, a negative test has no real diagnostic value.