To The Editors:
CLIA (Clinical Laboratory Improvement Act, 1988)-waived tests are by definition, "simple to operate and so accurate that error is very unlikely."1 They can be performed in the office without the requirement for a CLIA certification. In addition to waived tests for hemoglobin, dipstick urinalysis, blood glucose and urine pregnancy tests, there are at present three waived antigen detection tests for presumptive identification of group A streptococci from throat swabs. These are: OSOM™ Strep A Test (Wyntek Diagnostics, San Diego, CA), QuickVue™ In-Line Strep A Test (Quidel, San Diego, CA), and Binax Now™ (Binax Inc., Portland, ME). These individual tests cost between $3.50 and $4.50 to purchase and results are obtained in about 5 min.
We compared results of the OSOM™ test and the QuickVue™ test with that of concomitant sheep blood agar throat cultures after aerobic overnight incubation at 35°C. During a 3-week period ending May, 1997, 258 rayon-tipped throat swabs were analyzed for strep A antigen by a registered laboratory technologist while the patients and their parents waited in our office laboratory. For the 155 culture-negative specimens the specificity for both antigen detection tests was 100% (no false positives). For the 103 culture-positive specimens (40% of the total), the sensitivity was 95% and 87% for OSOM™ and QuickVue™ Strep A tests, respectively, almost identical with the respective package insert performance data.
Additional analysis of our data revealed that sensitivity was directly related to the number of colonies of group A strep. For 3+ and 4+ growth of group A strep colonies on the agar plate, both tests were 100% sensitive. For 1+ culture, OSOM™ Strep A test was 83% sensitive (5 of 6) whereas Quick Vue™ sensitivity was 33% (2 of 6). For 2+ culture, OSOM™ was 86% (24 of 28) sensitive whereas The QuickVue™ sensitivity was 72% (20 of 28). The negative predictive values for the OSOM™ and QuickVue™ Strep A tests were 97% and 92%, respectively.
The OSOM™ color immunochromatographic dipstick Strep A test performed at least as well as the best of the many rapid strep antigen detection tests that we evaluated in our office laboratory during the past 12 years. There are few reagent steps, the color endpoint is distinct, the 50 test strips are contained in a small easy-to-store cylinder and there is much less plastic for our biodisposable trash. Our nurses appreciated its simplicity and rapidity. The OSOM™ Strep A Test provides three levels of procedural controls with each test. Each kit contains additional standardized positive and negative controls which are to be tested twice per kit. According to the package inserts from all three waived tests mentioned above, for a negative result it is recommended that a routine throat culture be obtained.
Richard H. Schwartz, M.D.
Vienna Pediatric Associates; Vienna, VA
1. CLIA Q & A. Clinical Laboratory Management Association, Malvern, PA, and the American Association for Clinical Chemistry, Washington, DC, 1993.