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Comparison of the Performances of OSOM Ultra Strep A Test and
Standard Single Swab Culture Method, by Using Combined Culture Standard
(“CCS”; Single Swab Culture plus Broth Enhanced Pledget Culture) as the Gold Standard
In a field evaluation conducted by a clinical lab for a pediatric group practice, a total of 490 throat
swabs were collected from patients presenting with pharyngitis. Swabs were held in transport tubes
with modified Stuart’s transport media until testing.
Each swab was inoculated on a sheep blood agar plate, then tested by the OSOM Ultra Strep A Test.
The plate methods were the same as previously described. All pledgets from the transport tubes were
also tested following the enhanced pledget culture method described previously. A positive culture
from either the swab or the pledget was considered a Combined Culture Standard (CCS) positive.
Of 490 total specimens, 164 were found CCS positive and 326 were CCS negative, with a positive
rate of 33.5%. Of the 326 CCS negative specimens, 326 were also negative by the OSOM Ultra Strep
A Test, for a specificity of 100% (95% CI: 98.5 – 100%). Of the 164 CCS positive specimens, 161 and 157
were also positive by the standard single swab culture and the OSOM Ultra Strep A Test, respectively.
These equal to sensitivities of 98.2% (95% CI: 94.3 – 99.2%) and 95.7% (95% CI: 91.1 – 97.7%) for the standard
single swab culture and the OSOM Ultra Strep A Test, respectively. A statistical analysis comparing the
sensitivity of the standard single swab culture and the OSOM Ultra Strep A Test, using “The Difference
Between Two Independent Proportions” by Fleiss
(7)
, showed these two methods were not statistically
different (p = 0.3341).
The results are summarized below:
Test Results:
Single Swab Culture vs. CCS
CCS + CCS - Total
Culture + 161 0 161
Culture - 3 326 329
Total 164 326 490
OSOM Ultra Strep A Test vs. CCS
CCS + CCS - Total
Ultra + 157 0 157
Ultra - 7 326 333
Total 164 326 490
Sensitivity: 161 / 164 = 98.2%
Specificity: N/A
a
Sensitivity: 157 / 164 = 95.7%
Specificity: 326 / 326 = 100%
Sensitivity Analysis:
Test Method Sensitivity Specificity
SIngle Swab Culture 161/164 (98.2%) N/A
a
OSOM Ultra Strep A Test 157/164 (95.7%) 326/326 (100%)
p = 0.3341 N/A
a
a N/A, Not applicable, all culture positives are true-positive by definition.
Evaluation of the Performance of OSOM Ultra Strep A Test
by Using Standard Single Swab Culture Method as the Gold Standard
In a two site field evaluation, throat swabs were collected from patients presenting with pharyngitis. Each
swab was inoculated on a sheep blood agar plate, then tested by the OSOM Ultra Strep A Test. Plates
were incubated for 24 – 48 hours at 35˚C with 5 – 10% CO
2
. Presumptive GAS colonies were confirmed with
commercially available Strep A test kits. When the standard single swab culture method was used as the
gold standard, the combined results from these two sites showed that the OSOM Ultra Strep A Test had a
sensitivity of 96.4% (239/248; 95% CI: 93.0 – 97.9%), a specificity of 96.3% (524/544; 95% CI: 94.3 – 97.5%), and
an overall agreement of 96.3%. The results are summarized below:
Culture Density OSOM Ultra Strep A Results
4+ (predominant grwoth) 89/89 (100%)
3+ (>50 colonies) 90/91 (98.9%)
2+ (11 – 50 colonies) 45/46 (97.8%)
1+ (<10 colonies) 15/22 (68.2%)
Total Positive (Sensitivity) 239/248 (96.4%)
Negative (Specificity) 524/544 (96.3%)
a. N/A, Not applicable, all culture positives are true-positive by definition.
b. A statistical analysis using “The Difference Between Two Independent Proportions” by Fleiss
(7)
was used.
Statistical Analysis
b
for the Sensitivity of the Various Test Methods:
Test Method Sensitivity p
SSC A vs. SSC B 87 / 94 (92.6%) vs. 82 / 94 (87.2%) 0.3312
OSOM Ultra vs. SSC A 87 / 94 (92.6%) vs. 87 / 94 (92.6%) 0 .7811
OIA Max vs. SSC B 71 / 94 (75.5%) vs. 82 / 94 (87.2%) 0.0 581
OSOM Ultra vs. OIA Max 87 / 94 (92.6%) vs. 71 / 94 (75.5%) 0.0 021