Correlation of Electronically-Reported Varicella Laboratory Results to Clinical Diagnosis, Texas

BACKGROUND:   Texas requires health care providers, hospitals, laboratories, schools, and others to report patients with varicella (chickenpox) to the Texas Department of State Health Services (DSHS). Laboratory results for varicella are frequently reported without corresponding clinical information, making it unclear whether the patient has chickenpox, varicella immunity, or zoster/shingles. Identifying which laboratory results yield the most chickenpox cases could help reduce unnecessary work by public health surveillance staff.

METHODS:   DSHS identified electronically reported varicella laboratory results from the disease reporting registry for April and May, 2014. All labs that matched to patients with previously established diagnoses were excluded. A faxable form was mailed to each ordering provider, asking for the reason of testing. Using Excel, analysis on age group, type of test, and diagnosis/reason for testing was conducted.

RESULTS:   A total of 242 labs for unique patients were identified and had not been otherwise reported. There were 130 IgM results (54%), 82 IgG results (34%), 19 PCR results (8%), and 11 (5%) culture results. Of the 242 patients identified, DSHS received usable responses on 188 (78%). Patients between the ages of 20 and 29 constituted 35% of the lab results. Patients under 20 were the smallest age group at only 15%.  IgM and IgG were the predominant modes of testing at 52% and 39% respectively. The primary diagnosis or reason for testing was to test for immunity to varicella (64%), followed by shingles diagnosis (22%), chickenpox diagnosis (13%) and other reasons (1%). IgM was the most common test for those diagnosed with chickenpox and those with a diagnosis other than chickenpox (75% and 48% respectively). Another 43% of those not diagnosed with chickenpox were tested by IgG. Of the patients over 50, only 2 (6%) were diagnosed with chickenpox.

CONCLUSIONS:   While IgM is the most frequent test used for non-chickenpox diagnosis, it was also the most common test to diagnose chickenpox. The results indicate that IgG testing is predominantly used for testing purposes other than diagnosing chickenpox, especially in non-pediatric patients. Similarly, patients over the age of 50 are rarely diagnosed with chickenpox, regardless of test used. Eliminating these two categories (IgG results for patients over 20, all results for patients over 50) from follow up would reduce the burden from 188 lab results to 95 (a decrease of 50%) over two months, while only missing 2 chickenpox diagnoses.