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• This test does not differentiate between carriers and acute infection. Pharyngitis may be caused by
organisms other than Group A Streptococcus
(1, 2)
.
• A negative result may be obtained if the specimen is inadequate or antigen concentration is below
the sensitivity of the test.
• The American Academy of Pediatrics states
(4)
: “Several rapid diagnostic tests for GAS pharyngitis
are available ... The specificities of these tests generally are very high, but the reported sensitivities vary
considerably. As with throat cultures, the accuracy of these tests is most dependent on the quality
of the throat swab specimen, which must contain pharyngeal and tonsillar secretions, and on the
experience of the person who is performing the test. Therefore, when a patient suspected of having
GAS pharyngitis has a negative rapid streptococcal test, a throat culture should be obtained to
ensure that the patient does not have GAS infection.” It also states: “Cultures that are negative for
GAS infection after 24 hours should be incubated for a second day to optimize isolation of GAS.”
EXPECTED RESULTS
Approximately 19% of all upper respiratory tract infections are caused by Group A Streptococci
(5)
.
Streptococcal pharyngitis displays a seasonal variation and is most prevalent during winter and early
spring. The highest incidence of this disease is found in crowded populations such as military bases and in
school-age children
(6)
.
PERFORMANCE CHARACTERISTICS
In a multi-center evaluation, a total of 639 throat swabs were collected from patients presenting with
pharyngitis. Each swab was inoculated to a sheep blood agar plate, then tested by the OSOM Strep A
Test. Plates were incubated for 18 – 24 hours at 35° – 37°C at 5 – 10% CO
2
with a Bacitracin disk. Presumptive
GAS colonies were confirmed with commercially available Strep A testing kits.
Of the 639 total specimens, 464 were found to be negative by culture and 454 were also negative by the
OSOM Strep A Test, for a specificity of 97.8%. Of the 175 specimens found to be positive by culture, 168
were also positive by the OSOM Strep A Test, for a sensitivity of 96.0% . The 95% confidence intervals were
calculated to be 96.6 – 99.0% for specificity and 94.4 – 97.6% for sensitivity. Overall agreement between
culture and the OSOM Strep A Test was 97.3% (622/639). The results are summarized below:
• Just before testing, add 3 drops Reagent 1
(pink) and 3 drops Reagent 2 to the Test Tube
(the solution should turn light yellow).
• Immediately put the swab into the Tube.
• Vigorously mix the solution by rotating the swab
forcefully against the side of the Tube at least ten
(10) times. Best results are obtained when the
specimen is vigorously extracted in the solution.
In addition, the OSOM Strep A Test was used to confirm the identification of Group A Streptococcus on
blood agar plates. As a culture confirmation test, the OSOM Strep A Test was 100% sensitive (62/62) and
100% specific (39/39).
TEST PROCEDURE
Absorbent End Result Window Handle End
3 drops 3 drops
10x
Culture Classifications OSOM/Culture % Correct
Negative (Specificity) 454/464 97. 8 %
1+ (<10 colonies) 3/6 50.0%
2+ (11 – 50 col onies) 9/13 69.2 %
3+ (> colonies) 44/44 100%
4+ (predominant growth) 112/112 100%
Total Positive (Sensitivity) 168/175 96.0%
Total (Overall Agreement) 622/639 97. 3 %