Use of a High-Sensitivity Rapid Strep Test Without Culture Confirmation of Negative Results

Subjects

The Lahey Clinic’s Decision Support System tracks the clinical care delivered at these 2 locations, and we used it to identify all eligible patients. All those seen for pharyngitis in the outpatient clinics were identified by the International Classification of Diseases, Ninth Revision (ICD-9) codes for acute pharyngitis (462), sore throat (784.1), acute tonsillitis (463), and streptococcal tonsillitis (034.0). All patients seen with suppurative complications were identified by the ICD-9 codes for peritonsillar abscess (475) and retropharyngeal abscess (478.24). All patients seen with nonsuppurative complications were identified by the ICD-9 codes for acute rheumatic fever (390) and poststreptococcal glomerulonephritis (580.0). Diagnostic tests were identified by Current Procedural Terminology (CPT) codes for throat culture (87081) and the high-sensitivity antigen test (83518 or 87880).

Patients were separated into 2 cohorts, those seen during period 1 (April 1, 1994, to March 31, 1996) and those seen during period 2 (April 1, 1996, to March 31, 1998). All available medical records of those patients identified as having suffered a complication of GABHS were then reviewed in an effort to assess both coding accuracy and whether a false laboratory test (culture or antigen test) might have preceded the complication.

Microbiologic Investigations

Both cultures and the high-sensitivity antigen test were performed in the clinical microbiology laboratory in Burlington, Massachusetts, using methods previously described.¹⁹

Data Analysis

We used chi-square analysis to compare the testing strategies used in the 2 periods. The Fisher exact test was used to compare the complication rates between the 2 periods.

Results

Patients and Diagnostic Tests Performed

The Decision Support System identified a total of 30,036 patient encounters with a primary or secondary diagnosis of pharyngitis during the 4 years studied. We identified a total of 15,399 patients in period 1 and 14,637 in period 2 Table 1.

There was a statistically significant change in the diagnostic strategy used in each of the 2 periods. During period 1, throat culture was used in 65.6% of patient encounters, and no test was used in the remaining encounters. During period 2, the high-sensitivity antigen test was the only test used in 50.8% of patient encounters; no test was used in 33.7% of the encounters; throat culture was the only test used in 14.3% of patient encounters; and the high-sensitivity antigen test with culture confirmation was used in 1.3% of patient encounters (P <.001).

Suppurative Complication rates

There were no statistically significant differences in the rates of suppurative complications observed during the 2 time periods (P = .92).

During period 1, ICD-9 codes identified 34 patients with a principal diagnosis of peritonsillar abscess and 3 patients with a principal diagnosis of retropharyngeal abscess. During period 2, ICD-9 codes identified 35 patients with a principal diagnosis of peritonsillar abscess and 1 patient with a principal diagnosis of retropharyngeal abscess.

Charts were available for review for 72 of the 73 patients with suppurative complications (99%). The principal diagnosis was correctly coded on 71 of 72 of these patients (99%), with the one miscode being a patient who was initially thought to have a peritonsillar abscess but ultimately found to have a parotid tumor.

Of the 71 patients with suppurative complications whose charts were available, the median age of patients with a suppurative complication was 28.6 years (range = 8 to 64 years). Thirty-one of the 71 (44%) initially presented with the complication, so no antecedent diagnostic test was performed.

Of the 40 patients who had been seen before the development of the suppurative complication, 23 had received a streptococcal diagnostic test in a visit before the one during which the suppurative diagnosis was made (12, culture; 11, high-sensitivity antigen test). Of the 12 throat cultures performed, 3 (25%) were positive. Of 11 high-sensitivity antigen tests performed, 3 (27%) were positive.

Of the 40 patients who had been seen before the development of the suppurative complication, 27 had been previously treated with an antibiotic that would have been expected to eradicate GABHS: 12 with penicillin, 5 with amoxicillin/clavulanic acid, 4 with amoxicillin, 3 with clindamycin, 2 with erythromycin, and 1 with cephalexin.

Nonsuppurative Complication Rates

There were no statistically significant differences in the rates of nonsuppurative complication rates observed during the 2 time periods (P = .30).

The combination of ICD-9 codes and chart review revealed no new or recurrent cases of acute rheumatic fever during the 4 years studied. During period 1, ICD-9 codes identified 6 patients with a principal diagnosis of rheumatic fever. All 6 of these charts were available for review. After chart review, we found that 5 of these patients had a medical history of rheumatic fever, and one was a woman aged 65 years with rheumatoid arthritis. During period 2, ICD-9 codes identified 4 patients with a principal diagnosis of rheumatic fever. All 4 of these charts were available for review. After chart review, we found that 3 of these patients had a medical history of rheumatic fever. One was a boy aged 5 years who presented with arthritis in both knees, normal results on cardiac and electrocardiograph examinations, and was given a diagnosis of serum sickness following treatment with cefixime.