The humoral immune response after BCG vaccination in humans: consequences for the serodiagnosis of tuberculosis

The IgM and IgG response to BCG vaccination was investigated in 75 adults, tuberculin negative before vaccination, using an enzyme-linked immunosorbent assay with purified protein derivative (PPD) as antigen. The mean optical density (OD) increased significantly (p less than 0.001) in both immunoglobulin classes. Increase in at least one class was significant in 89% of the subjects. The observed increase in anti-PPD IgG was rather small but comparable to that seen in 17 newly diagnosed tuberculosis patients with negative direct smear [mean OD (SD): 0.59 (0.38) in vaccinated and 0.70 (0.48) in patients] but significantly lower (p less than 0.001) than that seen in 31 newly diagnosed patients with positive direct smear [mean OD (SD): 1.07 (0.67)]. With 55% of sera above the upper normal limit, smear positive patients differentiated (p less than 0.001) from vaccinated subjects (20% of positive sera) whilst smear negative patients (29% of positive sera) did not. We conclude that BCG vaccination induces a definite but small increase in anti-PPD serum IgM and IgG, which is likely to interfere when interpreting serological tests for the diagnosis of tuberculosis, especially in those patients who would most benefit from an early and fast diagnosis.