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BACKGROUND: Current pertussis diagnosis relies heavily on Polymerase Chain Reaction (PCR), although use of serology assays is increasing. To address changing diagnostic practices and significant numbers of PCR-positive cases not meeting the required clinical criteria, modifications to the current CSTE pertussis case definition are being considered. Proposed changes include making serology confirmatory (age ≥11 years) and loosening clinical requirements for PCR-positive persons ≥ 1 year old, mirroring modifications to the 2014 case definition for PCR-positive infants. Our objective is to determine the impact of these modifications on Minnesota case reporting.
METHODS: Pertussis notifications for persons ≥ 1 year with onset from 1/1/2011-10/17/2014 submitted to the Minnesota Department of Health (MDH) were classified as CSTE-confirmed or probable; those not meeting the definition or with incomplete clinical information were classified as not-a-case (NAC). All notifications were assessed to determine whether three proposed modifications would change the original classification. Modification 1 classified PCR-positive persons with any cough duration and ≥1 pertussis symptom as confirmed. Modification 2 classified PCR-positive persons with cough ≥14 days, regardless of symptoms, as confirmed. Modification 3 classified sero-positive persons aged ≥11 years with cough ≥14 days and ≥1 pertussis symptom as confirmed.
RESULTS: MDH received 6453 pertussis notifications during the study period. Of those, 5730 (89%) were classified as CSTE-confirmed or probable; 723 (11%) were NAC. Of the total notifications, 5004 (78%) were PCR-positive and 465 (9%) of these were NAC. Of 465 PCR-positive NAC reports, 324 (70%) had sufficient clinical data to be reclassified with alternative case definitions. Modification 1 reclassified 193/324 NAC reports to confirmed, while 131/324 were confirmed using Modification 2, increasing reportable cases by 3% and 2%, respectively. Eighty-eight sero-positive notifications were received. Of those, 77 (88%) were probable based on clinical symptoms alone, while 11 (12%) were NAC. Modification 3 reclassified 61/77 sero-positive cases to confirmed; 16/77 (21%) occurred among persons < 11 years. Sero-positive notifications represented 46/1154 (4%) cases among adults aged ≥ 20 years.
CONCLUSIONS: In Minnesota, the proposed case definitions could improve the sensitivity of pertussis surveillance by capturing those with milder presentations. Assuming clinical and laboratory practices don’t change, the modifications will only slightly increase reporting in MN and therefore would not drastically increase public health workload. States not investigating sero-positive notifications could be missing an important segment of adult disease. Clinically accurate assays are critical to avoid inclusion of false-positives as clinical criteria are relaxed.