# 4355
Yesterday, in the wake of the release of a study in the journal Pediatrics on the efficacy of RIDTs (Rapid Influenza Diagnostic Tests), there were a number of news reports that seemed to depict the results in vastly different ways.
We’ll look at the headline and lede of two of them.
Health Day News saw it this way:
Rapid Flu Test Most Accurate for Young Kids
HealthDay News
A widely available rapid influenza diagnostic test is good, but not perfect, in determining whether a child has the flu, a new study shows.
While Medpage Today reported:
Rapid Tests Fail at Ruling Out H1N1 in Kids
By Todd Neale, Staff Writer, MedPage Today
Published: February 15, 2010Rapid influenza tests have poor sensitivity for detecting pandemic H1N1 flu in children, two studies showed.
Confused?
One report says the test is `good but not perfect’, while the other states that it has `poor sensitivity’.
In truth, both headlines are correct . . . despite giving disparate impressions.
I’ll try to explain why.
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.
The various RIDTs are 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%
The study in question today is entitled:
Performance of a Rapid Influenza Test in Children During the H1N1 2009 Influenza A Outbreak
Andrea T. Cruz, MD, MPHa,b, Gail J. Demmler-Harrison, MDb,c,d, A. Chantal Caviness, MD, MPH, PhDa, Gregory J. Buffone, PhDc, Paula A. Revell, PhDc,
Here researchers evaluated the performance of RIDTs used at two different hospitals (that were capable of confirmatory rRT-PCR & Viral Culture Testing) over last summer. They were then able to determine the accuracy of one of 10 FDA approved RIDTs (BinaxNOW) .
A couple of excerpts from the abstract. Then some discussion.
Results . . . With rRT-PCR as the reference, overall test sensitivity was 45% (95% confidence interval [CI]: 43.3%–46.3%) and specificity was 98.6% (95% CI: 98.1%–99%). . . . . RIDT sensitivity was significantly higher in young infants and children younger than 2 years than in older children.
Conclusions The RIDT had relatively poor sensitivity but excellent specificity in this consecutive series of respiratory specimens obtained from pediatric patients.
In other words, if the test said someone had influenza, it was right almost all of the time (Note: This was during a time of high influenza activity).
But the test failed to detect influenza in roughly half the patients that were infected.
So the first headline Rapid Flu Test Most Accurate for Young Kids is true, since the report states that `sensitivity was significantly higher in young infants and children younger than 2 years.’
As is the second headline - Rapid Tests Fail at Ruling Out H1N1 in Kids – since overall, `The RIDT had relatively poor sensitivity.’
Contrary to popular belief, a 45% sensitivity level doesn’t mean that the test will be right less than half the time. That depends on the actual incidence of influenza in the community when the test is taken.
As we’ve discussed before, influenza makes up but a fraction of all of the ILI’s (Influenza-like-Illnesses) that circulate in a community. During the summer, that percentage drops into the low single digits, while at the height of flu season, it can approach 50%.
The other ILI culprits include Coronaviruses, metapneumovirus, parainfluenzavirus, respiratory syncytial virus (RSV), any of the myriad Rhinoviruses (Common cold), and some adenoviruses.
None of these are influenzas, and so none would be expected to test positive with the RIDT.
The following comes, again, from the CDC.
Accuracy Depends Upon Prevalence
The positive and negative predictive values vary considerably depending upon the prevalence of influenza in the community.
- False-positive (and true-negative) influenza test results are more likely to occur when disease prevalence is low, which is generally at the beginning and end of the influenza season.
- False-negative (and true-positive) influenza test results are more likely to occur when disease prevalence is high, which is typically at the height of the influenza season.
Clinical Considerations of Testing When Influenza Prevalence is Low
When disease prevalence is relatively low, the positive predictive value (PPV) is low and false-positive test results are more likely. By contrast, when disease prevalence is low, the negative predictive value (NPV) is high, and negative results are more likely to be true.
The interpretation of positive results should take into account the clinical characteristics of the case. If an important clinical decision is affected by the test result, the rapid test result should be confirmed by another test, such as viral culture or polymerase chain reaction (PCR).
Clinical Considerations of Testing When Influenza Prevalence Is High
When disease prevalence is relatively high, the NPV is low and false-negative test results are more likely. When disease prevalence is high, the PPV is high and positive results are more likely to be true.
So, to recap.
If there’s a lot of flu around, and you test positive, there’s a pretty good chance you really have the flu.
If you test negative . . . well, doctors are urged not to base a diagnosis solely on a negative test. Somewhere around 30% are probably false negatives.
And If there’s not much flu going around?
Even if you test positive, there’s a pretty good chance you don’t have the flu.
But a negative test (while not exclusionary) is substantially more likely to be true.
Obviously, given the ambiguity of all of this, a more accurate RIDT is something that doctors would be eager to get their hands on.
