1. Selective nonreactivity to purified protein derivative (PPD) antigen: five to ten percent of patients with active tuberculosis have negative tuberculin test results but a positive response to another antigen.
2. Mumps reactivity may remain after loss of PPD reactivity.
3. Patients with loss of PPD reactivity but positive reactions to control antigens can be boosted with a second tuberculin test seven to ten days later and may convert from a negative to a positive response.

Because of these findings, reactivity to control antigens with a negative response to PPD is not proof that the patient is not infected with tuberculosis. In addition, failure to respond to anergy tests does not mean the patient will not respond to PPD. Furthermore, anergy panels suffer from:

• Lack of standardized protocols for antigen selection, including types and numbers of antigens that need to be placed.
• Variability between lots of antigen available for purchase.
• Lack of uniform criteria for defining cutaneous reactivity.
• Regional variability in skin reactivity.
• Inability of anergy testing to predict the risk of progression to active tuberculosis.

In conclusion, a negative tuberculin test does not exclude active or latent tuberculosis in the presence of a reaction to other antigens placed as controls. The only useful result from the intracutaneous tuberculin test (PPD) is a positive result. The only skin test available at RML after January 15, 2003 will be the tuberculin test.

References: CAP Today, Pathology/Laboratory Medicine/Laboratory Management, Vol. 15, No. 8; August 2001.