Indian Pediatr 2019;56:311-313|
Utility of Detecting
sof Gene as Evidence of Streptococcus pyogenes
Infection in Acute Rheumatic Fever
Ravi N Mandal1,
Namita Ravi Kumar1
and Mohanaraj Ramachandran1
From 1Department of Pediatrics, Maulana
Azad Medical College, New Delhi, 2Case Western Reserve
University School of Medicine, Cleveland, Ohio, United States; and
3Department of Biotechnology, Institute of Genomics and
Integrative Biology, Council of Scientific and Industrial Research, New
Correspondence to: Dr Nikki Agarwal, Department of
Pediatrics, Maulana Azad Medical College, New Delhi, India.
Received: August 15, 2017;
Initial review: Janury 01, 2018;
Accepted: January 19, 2019.
Objectives: To determine the diagnostic accuracy
of polymerase chain reaction-based detection of sof gene compared
to throat swab culture for S. pyogenes infection in patients with
acute rheumatic fever and those with recurrence of rheumatic activity.
Methods: 40 patients between 3 to 18 years of age, with clinical
diagnosis of acute rheumatic fever or new activity in established
rheumatic heart disease were included. The amplicon of 228bp of sof
gene was detected using a polymerase chain reaction-based technique
and the results were compared with throat swab culture for
Streptococcus pyogenes. Results: 10 patients had a positive
throat swab culture and 11 had sof gene detected. The sensitivity
and specificity of the test was 100% and 96.7%, respectively compared to
throat swab culture (P=0.001). The positive predictive value and
the negative predictive value was 90.9% and 100% respectively.
Conclusions: Polymerase chain reaction-based detection of sof
gene provides an alternative to throat swab culture in diagnosing
activity in Acute Rheumatic Fever or established Rheumatic heart
Keywords: Diagnosis, Rheumatic heart disease, Streptococcal
vidence of Group A
streptococci (GABHS) infection is an essential criterion for the
diagnosis of Acute Rheumatic fever (ARF) . A positive throat swab
culture for GABHS is considered the gold standard for evidence of
previous streptococcal infection. However, it has been observed that
only 10-20% patients with ARF have positive throat swab culture [2,3].
In the last two decades, a few polymerase chain reaction (PCR) based
tests have been developed with sensitivity and specificity above 90% and
reporting times that range from 15 minutes to 3 hours, depending on the
technique [4-6]. One of these detects the amplicon of 228 base pair (bp)
of serum opacity factor (sof) gene, unique to invasive
species of S. pyogenes . This test has been entirely developed
in India with a turn-around time comparable to other PCR-based tests
. This study was conducted to determine the diagnostic accuracy of
PCR based technique of detecting sof gene in comparison with
throat swab culture.
This cross-sectional, observational hospital-based
pilot study was conducted between August 2012 and January 2014. Patients
between the ages of 3 to 18 years, with a clinical diagnosis of ARF or
recurrence of activity in previously diagnosed RHD (Modified Jones
criteria)  were consecutively enrolled. All patients with congenital
heart disease (CHD) were excluded. The protocol was approved by the
institutional ethics committee.
Carditis was defined clinically, by the presence of a
murmur in the aortic or mitral area, or development of a new murmur in
an established patient of RHD. High C- Reactive protein (CRP) levels was
defined as values above 6 mg/dL. Anti-streptolysin O (ASO) titers above
200 U/L were considered high as the hospital laboratory uses this cut
off for positive results and does not quantify further. Two throat swabs
were taken from the posterior pharynx and tonsillar surfaces. The first
was sent for culture on blood agar (reference test) and the second was
evaluated for the sof gene by the PCR based technique (index
test). A Chest X-Ray, 12-lead electrocardiogram and 2-Dimensional
Echocardiogram were also obtained.
The results of PCR-based sof gene detection
was assessed for performance against throat swab culture using the
diagnostic statistics: sensitivity, specificity, positive predictive
value (PPV) and negative predictive value (NPV). Statistical
significance was accepted at P<0.05, two tailed.
Forty patients met the eligibility criteria. Eleven
patients had recurrent ARF and 29 patients were new cases (Table
I). Throat swab culture grew GABHS in 10 patients. The sof
gene was detected in 11 patients. One patient presenting with isolated
chorea had a negative TSC but was positive for the sof gene. The
test demonstrated a specificity of 96.7%, and sensitivity of 100%, PPV
of 90.9% and NPV of 100.0%. The diagnostic accuracy was 97.5%.
TABLE I Distribution of the Defining Criteria in the Study Population (N=40)
New cases of ARF
No previous history of ARF
Presence of 2 major criteria
Presence of 1 major and 2 minor criteria
Recurrence of ARF
Previous history of ARF without
3 (7. 5)
residual heart disease
Previous history of ARF with residual heart disease
Presence of 2 major criteria
Presence of 1 major and 2 minor criteria
ARF: Acute rheumatic fever.
Many studies have reported that the inherent quality
of the PCR and superior extraction techniques makes them superior to
throat swab culture . The PCR-based sof gene test had
parameters of diagnostic accuracy that were comparable to the existing
PCR based tests [4-7]. This test detects the sof gene, which is a
bi-functional protein capable of binding the host-extracellular matrix
components of fibronectin and fibrinogen . The amplicon of 228 bp is
a unique characteristic of the sof gene which is found in the
invasive species of S. pyogenes and does not share homology with
other species of streptococcus (including Group C and G streptococci)
. The main advantage of this test is the ability to give results
within an hour, which could help in making clinical decisions in real
time . In addition, a positive sof gene test in chorea
demonstrates its clinical relevance in symptoms with long latent
periods, where throat swab culture are expected to be negative .
Another Indian PCR based test using the amplicon of 398 bp of mga
gene to detect GABHS has been developed; however, it has not been tested
in clinical studies .
We acknowledge a few limitations in this study. We
did not conduct this study in patients presenting with sore throat.
However, our aim was to determine the diagnostic accuracy of an
experimental test versus the gold standard in patients with
ARF/recurrence of rheumatic activity. Since the sample size was small it
is impossible to generalize these results to the community. However, the
high parameters of diagnostic accuracy, and short ‘turn-around’ time
(<60 minutes) of this PCR-based test, makes a compelling argument to
conduct large prospective community trials.
To conclude, our study shows that the PCR based
detection of the amplicon of the 228 bp of sof gene offers an
alternative way to detect S. pyogenes activity in patients
suspected of ARF or recurrence of activity in those with RHD.
Contributors: NA: performed the laboratory tests,
wrote and revised the manuscript; AM: wrote and revised the manuscript,
prepared the tables and images; SK, RNM and NRK: provided critique and
revised the manuscript; AM, AK: conceptualized the study and provided
critique to the manuscript; AK, MR: collected the data and provided
critique to the manuscript.
Funding: None; Competing interest: None
What This Study Adds?
A novel PCR-based test can detect
Group A b
hemolytic streptococcus with reasonable accuracy as compared to
throat swab culture.
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