Commercially available rapid strep tests are widely utilized in pediatrics across the care continuum. Traditionally the rapid test has detected antigens by enzyme or acid extraction from throat swabs. The specificity of these antigen tests is about 9%, with a sensitivity anywhere from 70-90%. Even if you miss only 10%, the sensitivity is low enough that a backup test has been required. In the past this was a culture, more recently a molecular assay that both required >24 hours to result in some cases.
Some of you will soon (if not already) be working in a clinical environment where a new rapid strep test is available. This new test is an isothermal nucleic acid amplification test and it does not require confirmation of a negative result.
How does it work?
You swab the back of the throat as you did before. Traditionally the rapid test swabs have had a red cap. The new “ID NOW™” test has a blue cap. You still have to actually swab the tonsils…
The assays are run one per machine and the time to result a positive is faster than the rapid influenza assay, about 4 minutes, and approximately 8-10 minutes for a negative result. The previous enzyme immune assay took about 5-6 minutes for a positive or negative result.
Why does it not require a “backup” test of a negative result?
ID NOW™ does not require confirmation of a negative result because the sensitivity and specificity are both significantly higher than antigen tests. Cohen et al., in a multi center trial of the aforementioned nucleic acid amplification test conducted at 10 sites, in primary care and Emergency Department settings, of over 480 patients. They compared the new swab to culture – and in cases where the results were discrepant- used PCR (highly accurate but expensive) to sort out the “true” result. Overall the population was mostly schools aged, all under 18, and had higher rates of strep with higher McIssac (aka Centor) scores. You should definitely check out the study – its an easy read – but the most important data if from the PCR adjudicated samples, which gives a better picture of the likely ability of this test. See the below figure for the test characteristics.
As you can see the overall sensitivity are both over 98%, with a negative predictive value of 99.4%. This has lead labs where the technology is available, after performing internal testing, to conclude that a negative backup result is not necessary because the new test will not miss a significant number of cases. Though the test costs more (one estimate is $87 for the rapid antigen test and $105 for the new molecular test) obviating the need to run backups will save money in the long run.
When will I see this where I work?
I don’t know. There are a lot of factors at play, some of which came to my attention after talking to some Pediatricians who work in the community.
- The machines cost money, and can only one one rapid strep at time. Hospital labs and Emergency Departments have the budget to run many at a time. Private Practice offices not so much… the only rate limiting step to doing the previous test is counter space and a person to run the test. You could run 4 or more at a time if you were efficient, which is important during ill-visit clinics and after hours settings where time is money.
- Know that if you are working in a big hospital and have the new test, other places in your community won’t. Therefore a negative rapid strep at an outside urgent care now does not mean the same thing as a negative rapid strep in your “new fancy pants test having” place. You should be prepared to address how this impacts care when you discuss things with families, especially in children with persistent fevers.
- Each facility – as Cohen and co-authors attest – should validate the machine in their setting. This takes time.
- You still have to contend with potentially missing ~1/200 patients with strep (NPV >99%). If your rate of rheumatic fever is low, and kids with false negative results follow up if they’re not getting better this is less of an issue.
- This study used PCR as a reference standard, which is costly, and not used everywhere. So if you look at the results without the extra adjudication step the sensitivity and specify are a little lower. ~96% and ~95% respectively.
What should I do next?
If you work somewhere where the next test is used/coming soon contact the lab and ask about how the decision was made. Also, read this study to get an idea of how a new test is vetted in an actual patient population.