Laboratory medicine journal
Original article
Sensitivity and Specificity of Rapid Antigen Test in Diagnosis of
Streptococcal Upper Respiratory Tract Infection
Nehad Abdulla, Mohamme Nafi Hamad
Department of Medical Microbiology, Faculty of Medical Laboratory Sciences, Al-Neelain
University, Khartoum, Sudan
Abstract
Background: Sore throat is one of the most common reasons for visits to family physicians.
While most patients with sore throat have an infectious cause (pharyngitis), fewer than 20%
have a clear indication for antibiotic therapy (i.e. group A β-hemolytic streptococcal
infection). Reducing the number of unnecessary antibiotic prescriptions given for common
respiratory infections has been recommended as a way to limit bacterial resistance.
Objective: This study aimed to detect the sensitivity and specificity of a rapid antigen
detection test (ICT) for diagnosis of throat infection caused by group (A) β- hemolytic
streptococci, compared with culture as the gold standard test.
Materials and methods: A total of 76 patients with new upper respiratory tract infections
accompanied by sore throat were assessed by a physician. Pharyngeal swabs were obtained
from all patients and both rapid test and throat culture were performed.
Results: Streptococcus pyogenes was identified in 22 out of 76 throat swabs (28.9%), of these
16 were positive by both ICT and culture. 19 (25%) were positive by ICT and also 19 (25%)
were positive by culture. The sensitivity of the ICT was 86.4% and specificity was 95%.
Conclusion: Our study supports new approaches to diagnosis of sore throat by using ICT of
streptococcus pyogenesas rapid, simple, and easy method in laboratory for any patients with
centor criteria, to avoid the misdiagnosis and unnecessary antibiotic use.
Keywords: Sore throat, pharyngitis, rapid streptococcal antigen test, group (A) β hemolytic
streptococcus.
Introduction
Acute pharyngitis constitutes a frequent
GAβ HS is necessary, because prompt
cause of visits and antibiotic prescribing in
antibiotic
primary healthcare facilities. Because the
faster subsidence of symptoms, prevention
signs and symptoms of pharyngitis due to
of early
group
streptococci
reduced spread of the strain to others, and
(GAβHS) pharyngitis can be non- specific,
it may be a useful tool to guide decisions
establishing an accurate clinical diagnosis
for antibiotic prescribing [1]. Culturing a
(A)
β-hemolytic
LMJ 2016; 2(1):6-9
therapy
and late
is
associated
complications
with
and
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Laboratory medicine journal
Original article
throat swab on a sheep blood agar plate
Streptococcus was identified by means of
has been accepted as the standard for the
standard techniques [4]. Rapid streptococcal
diagnosis of GAβHS pharyngitis for nearly
antigen assay (Hexagon Strep A, Germany)
five decades [2].
was performed according to the manu-
The major disadvantage of culturing throat
facturer's instructions.
swabs is the delay (overnight or longer) in
the rapid identification of GAβHS directly
Data analysis was performed by statistical
package for social sciences (SPSS). The
sensitivity and specificity of the rapid test
and the culture were calculated.
from throat swabs. All such rapid tests
Results
obtaining the results. Commercial rapid
antigen detection tests were developed for
involve
an
acid
extraction
step
to
solubilize GAβHS cell wall carbohydrate
and
to
identify
its
presence
by
an
immunologic reaction [3].
This study aimed to detect the sensitivity
and specificity of a rapid antigen detection
test (ICT) for diagnosis of throat infection
caused by group (A) β- hemolytic streptococci, compared with culture as the gold
standard test.
Materials and methods
A total of 76 patients presented with sore
throat to the ENT unit of Khartoum
teaching
hospital
during
the
period
between November 2012 and January 2013
were enrolled in the study. Informed
consent was obtained prior to enrollment.
For all patients, pharyngeal swabs were
taken
for
streptococcal
both
culture
antigen
and
assay.
rapid
Culture
specimens were plated on 5% sheep blood
agar plates and incubated aerobically for 2448
hours.
Group
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A
β
hemolytic
Demographic characteristics of patients are
listed in table (1). Children aged less than 10
years were represented the most affected
age group (53.9%) followed by patients aged
between 10-20
years
(27%).
Males
represented
35.5%
while
females
represented 64.5%. Regarding signs and
symptoms; all patients presented with sore
throat, while 84.2% suffered from swollen
tonsils or exudates and 80.3% presented
with fever. Streptococcus pyogenes was
isolated and identified in 19 (25%) of the 76
tested patients and rapid strep A test (ICT)
detect streptococcus pyogenes antigen in 19
(25%) swabs.
As shown in table (2) and out of 19 cultures
positive swabs 3 were negative by ICT, and
among 19 ICT positive 3 were culture
negative. The sensitivity of the ICT was
86.4% and specificity was 95%. Overall, the
positive and negative predictive values of
the rapid antigen detection (ICT) were 84
and 94 respectively.
Discussion
Sore throat or streptococcal pharyngotonsillitis has been a matter of medical concern
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Laboratory medicine journal
Original article
over the years, particularly because of its
potential for causing serious problems, such
as rheumatic fever and suppurative
complications. When a bacterial infection is
concerned to prevent unnecessary antibiotic
use, rapid streptococcal antigen test is a
reliable method to begin immediate
treatment. The prevalence of acute
pharyngo-tonsillitis caused by streptococcus
pyogenes is approximately 15% to 30%. This
percentage also varies from region to region
[5].
Table (1) Patients demographic characteristics, signs and symptoms and laboratory
testing results
Characteristic
No. of patients
Percent (%)
Age, years
< 10
41
53.9
10 – 20
24
31.6
> 20
Gender
11
14.5
Male
Female
Clinical findings
Fever
Sore throat
27
49
35.5
64.5
61
76
80.3
100
Tonsil swelling or exudates
64
84.2
Positive result
Culture
19
25.0
Rapid streptococcal antigen test
19
25.0
Table 2: Diagnostic value of rapid antigen detection (ICT) against culture
Variable
Culture
ICT
Total
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Total
Positive
Negative
Positive
16 (21%)
03 (04%)
19 (25%)
Negative
03 (04%)
54 (71%)
57 (75%)
19 (25%)
57 (75%)
76 (100%)
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Laboratory medicine journal
We found that, out of 76 patients tested the
prevalence of sore throat caused by the
streptococcus pyogenes is the 19 (25%) from
76 patients tested (Within the normal global
range), overall positive result 16 positive by
both rapid test and cultures. The use of a
rapid detection test, plus a bacterial culture
for the negative results, is currently
considered the most effective clinical
strategy [6]. In the current study, we found
3 ICT positive, culture negative samples
this mainly may due to antibiotic use or the
numbers of bacteria in swab is too scanty
(broth enrichment step not performed in
this study).
Although several studies have shown that
the rapid test, applied alone, does not have
sufficient sensitivity (75%- 85%) to
eliminate the need for cultures but has high
specificity of approximately 95 to 98% [7].
This means that the odds of a false positive
are lower than the odds of a false negative;
some researchers have demonstrated that
the more recently available rapid tests can
be more sensitive than bacterial culture,
particularly the immunoassay-based tests,
which can give results in a few minutes. In
our study, the sensitivity of rapid test (ICT)
was 86.4% and the specificity was 95%, this
can be more confident for positive results
than negative results.
Conclusion
Simple rapid antigen detection test (ICT) of
streptococcus pyogenes has higher specificity
Original article
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