Four-stage tuberculin testing in elderly subjects induces age-dependent progressive boosting

Area of Science:

• Immunology
• Geriatrics
• Infectious Disease Epidemiology

Background:

• Tuberculin skin test (TST) is a key diagnostic tool for tuberculosis (TB) infection.
• Elderly individuals often exhibit negative TST results, potentially due to diminished immune responses.
• The phenomenon of 'boosting' in TST reactions, where prior exposure enhances response to subsequent tests, is not fully understood in geriatric populations.

Purpose of the Study:

• To investigate the presence and extent of progressive boosting in tuberculin skin tests (TST) among individuals over 65 years of age.
• To evaluate the impact of sequential TST administrations on detecting latent tuberculosis infection in the elderly.
• To determine if standard TST protocols are sufficient for accurate TB screening in geriatric populations.

Main Methods:

• Administered four sequential tuberculin skin tests (5 TU, PPD) one week apart to 223 subjects aged 65 and older.
• Defined significant reactions as indurations ≥10 mm with an increase of ≥6 mm over the previous test.
• Analyzed reaction prevalence and size in relation to age and number of tests performed.

Main Results:

• Significant boosting was observed, with positive reaction rates increasing from 29% after the first test to 57% after the fourth (p<0.05).
• The percentage of positive reactors was inversely related to age (p<0.001), but this difference diminished with repeated testing.
• Even after four tests, detection rates varied by age group (46-70%), suggesting that standard protocols may miss a substantial proportion of positive reactors in the elderly.

Conclusions:

• Negative TST results in the elderly are often due to a waning immune response, which can be progressively restored by repeated testing (boosting).
• Standard two-step or even four-step TST protocols may be insufficient to detect all latent TB infections in individuals over 65.
• Careful interpretation of boosting reactions is essential to avoid misclassifying them as new infections (conversions) in geriatric TB screening.