Accuracy of Rapid Strep Testing in Patients
Accuracy of Rapid Strep Testing in Patients
Background: Some clinicians have questioned the accuracy of rapid diagnosis of group A streptococcal pharyngitis by commercial immunochemical antigen test kits in the setting of recent streptococcal pharyngitis, believing that the false-positive rate was increased because of presumed antigen persistence.
Methods: We studied 443 patients - 211 cases - who had clinical pharyngitis diagnosed as group A ß-hemolytic streptococcus infection in the past 28 days and compared them with 232 control patients who had symptoms of pharyngitis but no recent diagnosis of streptococcal pharyngitis. Our aim was narrowly focused to compare the rapid strep test with the culture method we used in our clinical practice.
Results: We found that the rapid strep test in this setting showed no difference in specificity (0.96 vs 0.98); hence, the assertion that rapid antigen testing had higher false-positive rates in those with recent infection was not confirmed. We also found that in patients who had recent streptococcal pharyngitis, the rapid strep test appears to be more reliable (0.91 vs 0.70, P < .001) than in those patients who had not had recent streptococcal pharyngitis.
Conclusions: The findings of this study indicate that the rapid strep test is both sensitive and specific in the setting of recent group A ß-hemolytic streptococcal pharyngitis, and its use might allow earlier treatment in this subgroup of patients.
Streptococcal antigen testing has been an important development in the office management of patients with pharyngitis. It has improved the appropriateness of antibiotic therapy and allowed earlier laboratory, evidence-based treatment of group A ß-hemolytic streptococcal pharyngitis. The main reason to treat streptococcal pharyngitis is to prevent rheumatic fever. Appropriate treatment can be started within the time it takes to get results from both rapid strep testing, which assesses the presence of streptococcal antigens, and laboratory culture. Earlier treatment is preferred by many patients and might also decrease transmission, symptom duration, and suppurative complications, such as peritonsillar abscess, retropharyngeal abscess, and cervical adenitis.
Clinical criteria alone, even with the use of various scoring systems, are not reliable enough to diagnose group A ß-hemolytic streptococcal pharyngitis. Rapid strep testing has been shown to be sensitive in the range of 70% to 95% compared with the most accurate culture methods. At our institution during the study interval our rapid strep test methods were shown by internal quality control data to detect 71% to 83% of positive cases, so it was our policy to back up negative rapid strep tests with a follow-up culture of the same swab, read at 24 and 48 hours. We thus had both methods available. When clinical judgment dictated, clinicians were able to choose one method rather than the other.
Lack of consensus within our shared group practice about the accuracy of rapid strep testing in the setting of recent streptococcal pharyngitis caused considerable confusion to patients and nursing staff. About one half of our physicians firmly asserted that the rapid strep test was not accurate when a patient had had recent streptococcal pharyngitis. They believed that the false-positive rate was substantially higher and that a 24- to 48-hour culture was the only test indicated in this situation. This group further believed that although antibiotics might kill the streptococcal bacteria, undegraded antigenic proteins could persist even in the absence of viable streptococcal bacteria in the pharynx. Destroying an organism's cell wall or metabolic capacity might not immediately remove the antigenic proteins from the area. Because the rapid strep test is based on the presence of antigen, not on antibody testing or culture, which tests for viable organisms, the possibility of a high false-positive rate seemed plausible to many members of our group. Other members of the group believed as strongly that the test was equally accurate in either clinical circumstance.
This difference of opinion caused logistical difficulties and increased the clinical workload, because the nurse performing the culture before the physician evaluation had to find a physician, assess that physician's belief about this clinical quandary, and assign the patient to that physician. Alternatively, the nurse could risk having to do a second throat culture, conflict with the physician, and further confusion by guessing which testing method the physicians working that particular shift would chose. As a result, patients often received conflicting and inconsistent messages about what test would be done and whether it would be accurate in their particular clinical circumstances. This confusion did not improve patient satisfaction and increased waiting time.
A search of 20 years of English MEDLINE abstracts and articles failed to find this question addressed in the medical literature. Because we are a consensus driven organization, we could not solve the problem by edict, so we chose to study this focused clinical question.
It was our aim to compare the rapid strep test with the two-step culture method we used in our clinical practice to determine whether there was a difference between these methods when assessing streptococcal pharyngitis in patients who had recently been treated for streptococcal pharyngitis and those who had not.
Although additional data collection, including serial antistreptolysin-O (ASO) testing, would have answered broader clinical questions, the focus of our study was narrowed to trying to answer only the specific question of whether the rapid strep test had a substantially higher false-positive rate in the group of patients recently treated for streptococcal pharyngitis.
Abstract and Introduction
Abstract
Background: Some clinicians have questioned the accuracy of rapid diagnosis of group A streptococcal pharyngitis by commercial immunochemical antigen test kits in the setting of recent streptococcal pharyngitis, believing that the false-positive rate was increased because of presumed antigen persistence.
Methods: We studied 443 patients - 211 cases - who had clinical pharyngitis diagnosed as group A ß-hemolytic streptococcus infection in the past 28 days and compared them with 232 control patients who had symptoms of pharyngitis but no recent diagnosis of streptococcal pharyngitis. Our aim was narrowly focused to compare the rapid strep test with the culture method we used in our clinical practice.
Results: We found that the rapid strep test in this setting showed no difference in specificity (0.96 vs 0.98); hence, the assertion that rapid antigen testing had higher false-positive rates in those with recent infection was not confirmed. We also found that in patients who had recent streptococcal pharyngitis, the rapid strep test appears to be more reliable (0.91 vs 0.70, P < .001) than in those patients who had not had recent streptococcal pharyngitis.
Conclusions: The findings of this study indicate that the rapid strep test is both sensitive and specific in the setting of recent group A ß-hemolytic streptococcal pharyngitis, and its use might allow earlier treatment in this subgroup of patients.
Introduction
Streptococcal antigen testing has been an important development in the office management of patients with pharyngitis. It has improved the appropriateness of antibiotic therapy and allowed earlier laboratory, evidence-based treatment of group A ß-hemolytic streptococcal pharyngitis. The main reason to treat streptococcal pharyngitis is to prevent rheumatic fever. Appropriate treatment can be started within the time it takes to get results from both rapid strep testing, which assesses the presence of streptococcal antigens, and laboratory culture. Earlier treatment is preferred by many patients and might also decrease transmission, symptom duration, and suppurative complications, such as peritonsillar abscess, retropharyngeal abscess, and cervical adenitis.
Clinical criteria alone, even with the use of various scoring systems, are not reliable enough to diagnose group A ß-hemolytic streptococcal pharyngitis. Rapid strep testing has been shown to be sensitive in the range of 70% to 95% compared with the most accurate culture methods. At our institution during the study interval our rapid strep test methods were shown by internal quality control data to detect 71% to 83% of positive cases, so it was our policy to back up negative rapid strep tests with a follow-up culture of the same swab, read at 24 and 48 hours. We thus had both methods available. When clinical judgment dictated, clinicians were able to choose one method rather than the other.
Lack of consensus within our shared group practice about the accuracy of rapid strep testing in the setting of recent streptococcal pharyngitis caused considerable confusion to patients and nursing staff. About one half of our physicians firmly asserted that the rapid strep test was not accurate when a patient had had recent streptococcal pharyngitis. They believed that the false-positive rate was substantially higher and that a 24- to 48-hour culture was the only test indicated in this situation. This group further believed that although antibiotics might kill the streptococcal bacteria, undegraded antigenic proteins could persist even in the absence of viable streptococcal bacteria in the pharynx. Destroying an organism's cell wall or metabolic capacity might not immediately remove the antigenic proteins from the area. Because the rapid strep test is based on the presence of antigen, not on antibody testing or culture, which tests for viable organisms, the possibility of a high false-positive rate seemed plausible to many members of our group. Other members of the group believed as strongly that the test was equally accurate in either clinical circumstance.
This difference of opinion caused logistical difficulties and increased the clinical workload, because the nurse performing the culture before the physician evaluation had to find a physician, assess that physician's belief about this clinical quandary, and assign the patient to that physician. Alternatively, the nurse could risk having to do a second throat culture, conflict with the physician, and further confusion by guessing which testing method the physicians working that particular shift would chose. As a result, patients often received conflicting and inconsistent messages about what test would be done and whether it would be accurate in their particular clinical circumstances. This confusion did not improve patient satisfaction and increased waiting time.
A search of 20 years of English MEDLINE abstracts and articles failed to find this question addressed in the medical literature. Because we are a consensus driven organization, we could not solve the problem by edict, so we chose to study this focused clinical question.
It was our aim to compare the rapid strep test with the two-step culture method we used in our clinical practice to determine whether there was a difference between these methods when assessing streptococcal pharyngitis in patients who had recently been treated for streptococcal pharyngitis and those who had not.
Although additional data collection, including serial antistreptolysin-O (ASO) testing, would have answered broader clinical questions, the focus of our study was narrowed to trying to answer only the specific question of whether the rapid strep test had a substantially higher false-positive rate in the group of patients recently treated for streptococcal pharyngitis.
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