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• The OSOM Ultra Strep A Test should be used only with throat swab specimens. The use of swab
specimens taken from other sites or the use of other samples such as saliva, sputum or urine has
not been established.
• This test does not differentiate between carriers and acute infection.
• Pharyngitis may be caused by viral or bacterial pathogens other than Group A Streptococcus
(1, 2 )
.
• If the test result is inconsistent with the clinical symptoms, a second throat swab should be collected
for repeat testing.
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
Summary
The study results in this section show that the sensitivities of the OSOM Ultra Strep A Test and the standard
single swab culture method are not statistically different. The study also compared the OSOM Ultra Strep
A Test to Rigorous Gold Standard and another commercially available rapid test, Inverness Medical –
BioStar’s Strep A OIA
®
Max Test***.
Comparison of the Performances of OSOM Ultra Strep A Test, Strep A OIA Max Test and Standard Single
Swab Culture Method, by Using Rigorous Gold Standard (“RGS”; Multiple Sample Swab Culture plus Broth
Enhanced Pledget Culture) as the Gold Standard
In a hospital clinical lab field evaluation, two swabs (A and B) were collected from each of 302 patients
presenting with pharyngitis. Swabs were held in transport tubes with Stuart’s modified transport medium
until testing. Swabs A and B from each patient were inoculated on separate selective sheep blood agar
plates. One swab (Swab A) was then tested by the OSOM Ultra Strep A Test and the other swab (Swab
B) was tested by the Strep A OIA Max Test. Plates were incubated for 24 – 48 hours at 35˚C with 5 – 10%
CO
2
. All pledgets from the transport tubes were placed aseptically in modified Todd Hewitt Broth (THB)
for 16 – 24 hours at 35˚C with 5 – 10% CO
2
. After the initial incubation, the inoculated THB was sub-cultured
on SXT agar plates and incubated for 24 – 48 hours at 35˚C with 5 – 10% CO
2
. All presumptive GAS colonies
were confirmed with commercially available Strep A test kits. A positive culture from either one of the
two swabs or the pledget was considered a Rigorous Gold Standard (RGS) positive.
Of 302 total patients sampled, 94 were found RGS positive and 208 were RGS negative, with a positive
rate of 31.1%. The sensitivity of the OSOM Ultra Strep A Test, 92.6% (95% confidence interval (CI): 84.8 – 96.0%)
was the same as the corresponding standard single swab culture, 92.6% (95% CI: 84.8 – 96.0%). In these
studies, the two test methods’ sensitivities were not statistically different (p = 0.7811). The sensitivity of the
OSOM Ultra Strep A Test was 92.6% and the sensitivity of the Strep A OIA Max Test was 75.5%, with p value
of 0.0021. The results are summarized below:
Individual Test Results Compared to the RGS:
Swab A - Culture vs RGS
RGS + RGS - Total
Culture + 87 0 87
Culture - 7 208 215
Total 94 208 302
Sensitivity: 87 / 94 = 92.6%
Specificity: N/A
a
Swab A - OSOM Ultra Strep A Test vs. RGS
RGS + RGS - Total
Ultra + 87 15 102
Ultra - 7 193 200
Total 94 208 302
Sensitivity: 87 / 94 = 92.6%
Specificity: 193 / 208 = 92.8%
Swab B - Culture vs RGS
RGS + RGS - Total
Culture + 82 0 82
Culture - 12 208 220
Total 94 208 302
Sensitivity: 82 / 94 = 87.2%
Specificity: N/A
a
Swab B - Strep A OIA Max Test vs RGS
RGS + RGS - Total
OIA + 71 6 77
OIA - 23 202 225
Total 94 208 302
Sensitivity: 71 / 94 = 75.5%
Specificity: 202 / 208 = 97.1%
Summary of the Test Method Sensitivity and Specificity When Compared to the Rigorous Gold Standard:
Test Method Sensitivity Specificity
Single Swab Culture A (SSC A) 87 / 94 (92.6%) N/A
a
Single Swab Cuture B (SSC B) 82 / 94 (87.2%) N/A
a
OSOM Ultra Strep A Test (Swab A) 87 / 94 (92.6%) 193 /208 (92.8%)
Strep A OIA Max Test (Swab B) 71 / 94 (75.5%) 202 / 208 (97.1%)