BCG vaccine may confound PPD readings, but several studies indicate that PPD can still be a useful screening tool for tuberculosis infection after vaccination. A Brazilian case-control study found that reactions by those BCG recipients later exposed to TB were significantly greater than those with no TB exposure.3 The study noted that 47.5% of exposed children (defined as those with a household contact) had PPD readings of >10 mm, compared with just 3.6% of control children. In a Quebec cohort of 1198 foreign-born children and young adults, prior BCG vaccination could account for 50% of PPDs with induration of 5 to 9 mm, but only 4% of reactions 10 mm or greater. This study also showed that patients from countries with a high or moderate incidence of TB were more likely to have reactive PPDs than those from countries of low incidence, suggesting that exposure to TB accounts for some of the positive PPDs.4
Where it is available, the QuantiFERON-TB Gold test may be used in place of, or in addition to, the PPD for patients who are known to have received a BCG vaccine. This blood test detects interferon-gamma in the serum of people sensitized to Mycobacterium tuberculosis. Because the test is specific to proteins found in M tuberculosis, there is no cross-reactivity with BCG. A Japanese study of 216 BCG-vaccinated individuals showed interferon-gamma assays to be 98.1% specific. The same study reported 89.0% sensitivity for the combination of 2 interferon-gamma assays among 118 TB culture-confirmed individuals.5 A published report estimated the cost to the health care system per patient tested by a single interferon-gamma release assay as $33.67, compared with approximately $11 for PPD testing.6
TABLE
PPD reactions >10 mm when BCG was given during and after infancy
| RECEIVED BCG | NO BCG | RR | (95% CI) | |
|---|---|---|---|---|
| Given in infancy | ||||
| Timing of PPD unspecified | 22.3% | 19.2% | 1.16 | (1.09–1.23) |
| PPD less than 15 yrs since BCG | 12.6% | 5.2% | 2.4 | (2.00–2.97) |
| PPD more than 15 yrs since BCG | 47.2% | 41.0% | 1.2 | (1.09–1.22) |
| Given after infancy | ||||
| Timing of PPD unspecified | 35.6% | 17.4% | 2.08 | (1.89–2.21) |
| PPD less than 15 yrs since BCG | 29.1% | 2.9% | 10 | (5.29–18.99) |
| PPD more than 15 yrs since BCG | 37.6% | 47.8% | 0.8 | (0.74–0.85) |
| PPD, purified protein derivative; BCG, bacille Calmette-Guérin; RR, relative risk; CI, confidence interval | ||||
Recommendations from others
While the US Preventive Services Task Force (USPSTF) does not make a specific recommendation regarding PPD readings after BCG vaccine, it does recommend screening high-risk populations. The USPSTF further notes that reactions >10 mm should not be attributed to prior BCG vaccine.7
The Centers for Disease Control and Prevention (CDC) and American Thoracic Society joint statement recommends against altering guidelines for testing and interpretation among BCG recipients.8 In 2005, the CDC recommended the QuantiFERON-TB Gold test be used under the same indications as the PPD, noting its potential benefit among those previously immunized with BCG.9