Comparison of Illumigene Group A Streptococcus Assay with Culture of Throat Swabs from Children with Sore Throats in the New Zealand School-Based Rheumatic Fever Prevention Program
Publication Date: 11/11/2015
Conference or Journal: Journal of Clinical Microbiology
Author(s): Arlo Upton, Liselle Bissessor, Elizabeth Farrell, Stanford T. Shulman, Xiaotian Zheng, Diana Lennon
Group A streptococcal (GAS) pharyngitis is a particularly important condition in areas of New Zealand where incidence of acute rheumatic fever remains unacceptably high. Prompt diagnosis and treatment of GAS pharyngitis is a cornerstone of the Rheumatic Fever Prevention Programme, but this is hindered by the turn-around time of culture. Tests with excellent performance and rapid turn around time are needed. For this study throat swabs (Copan ESwabs) were collected from school children self-identifying with a sore throat. Samples were tested by routine culture and illumigene GAS assay using Loop-Mediated Isothermal Amplification. Discrepant results were resolved with retesting the same specimen and using an alternative molecular assay. Seven-hundred and fifty seven throat swabs were tested by both methods. illumigene performance using culture on blood agar as the gold standard and following discrepancy analysis was: sensitivity (82% and 87%), specificity (93% and 98%), positive predictive value (61% and 88%), and negative predictive value (97% and 97%). In our unique setting of a school-based throat swabbing programme the illumigene did not perform quite as well as previous reports. Despite this, its improved sensitivity and rapid turn-around compared with culture is appealing.
Group A streptococcal (GAS) throat infections are particularly significant as a sub-group of people (typically children) will develop acute rheumatic fever (ARF) or acute post streptococcal glomerulonephritis. Although treatment of GAS pharyngitis infection with appropriate antibiotics markedly reduces the risk of ARF, New Zealand (NZ) continues to have high rates of ARF compared with other developed nations (1). In 2011, the NZ Government announced a target for a reduction in the national incidence of ARF by two thirds, from 4.2 per 100,000 people in 2011, to 1.4 per 100,000 people by 2017 (2). To achieve this goal, the Rheumatic Fever Prevention Programme (RFPP) was initiated with an emphasis on timely detection and treatment of GAS pharyngitis. The programme focuses improved access of school-aged children to throat swabbing services who are considered to be at the highest risk of ARF, specifically Māori and Pacific children residing in areas of high socioeconomic deprivation (3). A large component of the intervention has been the school-based throat swabbing clinics for children who self-present during the school day with sore throats. As the signs and symptoms of bacterial and viral pharyngitis overlap and differentiation on clinical grounds is difficult, throat swab for culture is the current gold standard for diagnosing GAS pharyngitis (4).
However, it is an imperfect test for use in school-based programmes as results are not available until the following day at the earliest (and up to 72 hours after swabbing), and widespread swabbing such as seen in this programme puts considerable resource strain on community laboratories. Given these limitations, tests with more rapid turn-around times and that offer laboratory efficiencies are worth investigating. The illumigene Group A Streptococcus assay (Meridian Bioscience, Inc., Cincinnati, OH) uses Loop-Mediated Isothermal Amplification (LAMP) technology to detect the GAS pyrogenic exotoxin B (speB) gene, and it has FDA clearance for diagnosing GAS pharyngitis. The assay is semi- automated and has a turn-around time of less than an hour. The aim of this laboratory based study was to assess the performance of the illumigene GAS assay for the detection of GAS in throat swabs of symptomatic children in the unique setting of the school based component of the RFPP.
MATERIALS AND METHODS
The study was an extension of the already established throat swabbing service in Auckland school children at risk of ARF and did not deviate from routine clinical care. The study ran at a single South Auckland primary school and participating children (aged five to eleven years) were those who had already consented/assented to the school-based public health intervention. Each class room is visited by a health worker daily (Monday to Friday) and children are asked to self-identify as having a sore throat. Those children then have a throat swab collected for routine microbiological culture. As there was no deviation from routine clinical care a specific consent or new ethics approval were not required for this study. Throat swabs were collected using Copan ESwabs (Copan Diagnostics, Inc., Corona, CA). The ESwab is a nylon-flocked swab designed to minimize entrapment of the specimen. Swabs were collected and immediately placed into 1ml liquid Amies transport media. Although this was a deviation from the illumigene manufacturer’s instructions, the ESwab’s design facilitates elution of bacteria collected on the swab into the liquid transport media. Swabs were kept at room temperature and transported to the laboratory, arriving up to eight hours following collection. On receipt in the laboratory, specimens were processed by trained laboratory technicians. The sample was vortexed for ten seconds before 50μl of transport media was removed for illumigene, and then 50μl of transport media for culture. Culture methods were routine with inoculation onto Columbia Sheep Blood Agar with 3% Salt (Fort Richard, Auckland) media and incubation in CO2 at 37°C for up to 48 hours. Presumptive GAS colonies (based on colonial morphology) were identified by the Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI TOF MS, Bruker Daltonics, Biotyper Version 3.1) with a score of > 2.0, and were reported qualitatively. Isolation of group C/G streptococci was also reported (identification by MALDI TOF MS). With the exception of the inoculation with transport media rather than with the swab, illumigene testing was performed according to the manufacturer’s instructions.
Briefly, 50μl of transport media was added to the sample preparation tube and vortexed for ten seconds. Following vortexing, ten drops of the specimen were transferred to a heat treatment tube and incubated at 95°C for ten minutes. Then 50μl of lysate was transferred to the test and control chambers. The test device was then inserted into the illumipro-10 incubator/reader and amplification initiated. After 40 minutes the amplified product was detected by the presence of turbidity and read by the illumipro-10.
The extracted DNA from the ESwab eluate with discordant results (illumigene positive, culture negative) was tested at the Ann & Robert H. Lurie Children’s Hospital of Chicago by real time PCR performed on the Light-Cycler instrument with Roche analyte-specific reagents for GAS (Roche Diagnostics, Indianapolis, IN), as previously reported (5). A 198-bp fragment of the ptsI (phosphotransferase) gene of GAS was amplified and detected.
Culture positive/illumigene negative samples were retested by both methods. If the illumigene remained negative a dilution (0.5 McFarland) was made from a representative GAS colony and tested by illumigene. It was intended that culture negative/illumigene positive samples underwent repeat culture but this was not universally done due to error.
Past studies have demonstrated excellent sensitivity of the illumigene test (5–7). The expected 100 positive GAS rate was 14% (from school swabbing data). Therefore in order to obtain positive swabs it was calculated that 720 swabs would be required.
Sensitivity, specificity, positive and negative predictive values were calculated first using culture as the gold standard. However, it is acknowledged that the current gold standard is culture from a swab, not liquid transport media. Then, after discrepancy testing, they were recalculated using an estimate of GAS status where a true positive result was either culture positive or positive by both molecular assays. A true negative result was where the culture was negative and the specimen was either illumigene negative or positive by illumigene but negative by PCR.
A total of 757 throat swabs were tested by both illumigene and routine culture. Ninety-two (12.2%) were positive for GAS by culture, 57 (7.5%) were positive by culture for group C/G streptococcus, and 608 (80.3%) were culture negative. A total of 124 (16.4%) swabs were positive by illumigene, and of these 75 (60.5%) were also culture positive. A total of 633 (83.6%) were negative by illumigene, and of these 616 (97.3%) were also culture negative (Table 1). Using culture as the gold standard the prevalence of GAS was 12%, and the sensitivity, specificity, positive predictive value, and negative predictive values for the illumigene GAS assay were 82%, 93%, 61%, and 97%, respectively.
There were 49 throat swabs negative for GAS by culture but illumigene positive. Two were found to be culture positive on repeat testing. Of the remaining 47 swabs, the extracted DNA from 46 was further tested and GAS DNA was identified in 34 samples, one of which also had group C/G DNA identified. Eight samples had no streptococcal DNA while four had only group C/G DNA identified.
There were seventeen throat swabs GAS negative by illumigene and positive by culture. Of these, seven remained negative by illumigene on repeat testing of the stored transport media, while ten were positive when illumigene was repeated. Of the seven that remained negative by illumigene, six had GAS identified on re-culture. All of these were positive by illumigene when tested using 50μL of a 0.5 McFarland suspension.
Following discrepancy analysis the recalculated GAS prevalence was 17%. The recalculated sensitivity, specificity, and positive and negative predicative values for both culture and illumigene were 73%, 100%, 100%, and 95%, and 87%, 98%, 88%, and 97%, respectively (Table 2).
This study took place in the unique environment of a developed country tackling pockets of high incidence of ARF, with targeted throat swabbing programme for GAS in children self-identifying as having a sore throat. We found illumigene GAS to have superior sensitivity to culture for the detection of GAS in throat swabs, but specificity was reduced. Over-all illumigene detected more GAS although seventeen specimens were negative despite positive culture on initial testing. The finding of ten samples initially illumigene negative but positive on repeat testing suggests an issue with test reproducibility as well as sensitivity. The GAS isolates from six throat swabs that were repeatedly illumigene negative did test positive when a diluted solution was made from the isolate from the culture plate. Thus, the false negative results may have been due to the bacterial load in the sample being lower than the lower limit of detection, which is reported in the package insert to be 400 CFU (8). The variability seen among some specimens may be due to sampling error with bacteria being unevenly spread throughout the sample.
However, specimens were vortexed before sampling to reduce the likelihood of this. It is possible that use of the liquid media led to dilution of GAS, but the illumigene false negative samples were culture positive indicating that the bacterial load was sufficient for culture. However, the use of the transport media rather than swab was a deviation from the manufacturer’s recommendations and we acknowledge that this may have impacted on the sensitivity of the test. We do not expect the delay of up to eight hours to have impacted on illumigene results as the manufacturer states that the samples are stable at room temperature for up to 48 hours from swabbing to performance of illumigene (8) although it is acknowledged that this based on rayon swabs. Following discrepancy analysis, the illumigene sensitivity (87%) remained inferior to recent publications which have reported sensitivities of greater than 98% (5–7).
Current guidelines recommend back-up throat swab culture for negative rapid antigen detection tests for GAS as their sensitivity and NPV is insufficient to rule out GAS pharyngitis (4). Here we report a NPV for illumigene of 97%. While it could be argued that samples with negative illumigene results should also have culture, this would be impractical in our laboratory given the sheer volume of samples received and the on-going resource constraints. Not unusually for a molecular assay the illumigene was less specific than culture, with a PPV of 87.9% (95% CI 80.5 – 92.8). Again the performance is not as robust as previously reported (specificities > 96%) (5–7). We hypothesize that the lower PPV seen here was due, in part, to our study population of children who were well enough to be at school and self-identified with a sore throat.
In contrast, the previously published studies examined throat swabs from children presenting to the emergency room with pharyngitis. Thus, the pre-test probability in the school population could be lower because some are likely to be carriers with a lower GAS burden than in emergency room patients presenting with more acute and dramatic symptoms. A further explanation for false positive results is that speB may not be specific to GAS; isolates of group C/G streptococcus and Streptococcus anginous group have been identified as carrying speB in India (9). Interestingly, four of our samples that were illumigene positive and culture negative were identified as Streptococcus C/G by PCR. Lastly, the sensitivity of our culture may have been impacted by the volume of transport media used (50μL) and the incubation in CO2 (10).
The clinical impact of false positive tests in the context of the RFPP may be less concerning than in routine primary care (11). In all likelihood, children in the RFPP are being ‘treated’ for GAS pharyngeal colonisation as well as infection (as they are in routine primary care (4, 12)). Although this is ‘unnecessary’ antibiotic therapy, it may be reducing carriage and transmission of GAS in the community, and thus among children at high risk of ARF the benefits of additional antibiotics may outweigh the slight risks. The significant advantage the illumigene assay offers RFPP, in addition to enhanced sensitivity, is considerably improved turn-around time enabling same-day reporting of results and antibiotic prescription, with consequent reduced period of infectivity. However, use of illumigene or other molecular assays does not result in an isolate on which antibiotics susceptibilities can be performed.
This has little practical impact in our setting as GAS is universally susceptible to penicillins and cephalosporins, and macrolides are only employed for those with significant beta-lactam allergy. In addition, erythromycin resistance remains low (approximately 4%) in our community.
This study has some limitations. Firstly, our methods deviated from the illumigene manufacturer’s recommendations in that the transport media tested rather than the swab. However, the FDA submission does report acceptable performance with Liquid Amies transport media. Discrepancy PCR testing was done on extracted eluate and we cannot rule out specimen contamination. We did not report GAS growth quantitatively and so were not able to establish the impact of bacterial load on illumigene sensitivity. The DNA from one specimen was not referred for further molecular testing in error. Clinical data were not collected and so could not evaluate the impact of severity of illness or concurrent antibiotic administration on test performance.
In summary, the illumigene GAS test offers the community laboratory molecular test for detection of GAS in throat swabs with rapid turn-around time. The assay identifies more true positive GAS results at the cost of a slight drop in specificity (100 to 98%) when compared to culture.