Tuberculin Skin Test in Patients with Cough
Yes, you can and should perform a tuberculin skin test (TST) on a patient with a cough—in fact, the presence of cough is a key indication for TB screening and does not contraindicate TST administration. The cough itself is one of the primary symptoms that should prompt diagnostic evaluation for tuberculosis, which includes TST as part of the workup. 1
When TST Should Be Performed in Patients with Cough
Any patient with persistent cough or symptoms compatible with tuberculosis (weight loss, anorexia, fever, night sweats) should undergo diagnostic measures that include tuberculin skin testing, along with history, physical examination, chest radiograph, and sputum examination. 1 The CDC guidelines explicitly state that a diagnosis of tuberculosis should be considered for any patient with persistent cough, and TST is part of the standard diagnostic battery. 1
Key Clinical Scenarios:
Patients presenting with cough of any duration in high TB prevalence settings should be screened with TST, as TB prevalence is similar whether patients have cough for 1,2,3, or 4 weeks. 2
For HIV-infected patients with cough plus any additional symptoms (fever, night sweats, hemoptysis, or weight loss), TST should be performed immediately as part of the diagnostic workup, since these symptoms significantly increase the likelihood of active pulmonary TB. 2
Healthcare workers with persistent cough (lasting ≥3 weeks), especially with other TB-compatible symptoms, should be evaluated promptly with TST and should not return to work until TB is excluded or they are on therapy and determined to be noninfectious. 1
Critical Understanding: TST Limitations in Active Disease
A negative TST does not rule out active tuberculosis disease. This is a crucial pitfall to avoid. 1 The guidelines emphasize that TST should never be used to exclude the possibility of active TB among persons for whom the diagnosis is being considered, even if the skin test is negative. 1
Why Negative TST Can Occur with Active TB:
False-negative skin tests are common in patients with active TB, particularly those who are immunocompromised or HIV-infected. 1
The likelihood of false-negative TST increases as HIV infection advances, making clinical judgment paramount over test results. 1
Approximately 23% of tuberculosis patients have negative skin tests (<5mm induration), with higher rates in those with risk factors—50% in immunosuppressed patients and 61% in HIV-infected individuals. 3
Proper TST Administration and Interpretation
The Mantoux technique (0.1 mL of PPD containing 5 tuberculin units injected intradermally) must be used, producing a pale wheal of 6-10 mm diameter. 4 The test must be read by trained personnel between 48-72 hours after injection, measuring only induration (not erythema). 4
Interpretation Based on Risk Status:
For high-risk individuals (HIV-infected, close TB contacts, immunosuppressed): ≥5 mm induration is positive. 4, 1
For moderate-risk individuals: ≥10 mm induration is positive. 4
For low-risk individuals with no known risk factors: ≥15 mm induration is positive. 4
Infection Control During TST Administration
If the patient with cough is suspected of having infectious TB, appropriate respiratory precautions must be taken during TST administration. 1 The patient should:
- Be placed in respiratory isolation if active TB is suspected. 5
- Cover their mouth and nose with tissues when coughing. 1
- Remain in isolation areas and not return to common waiting areas. 1
Healthcare workers administering TST to patients with suspected infectious TB should wear respiratory protection. 1
Complete Diagnostic Approach
TST should be part of a comprehensive diagnostic evaluation, not used in isolation. The complete workup for a patient with cough suspicious for TB includes: 1
- Medical history focusing on TB risk factors and exposure
- Physical examination
- Tuberculin skin test
- Chest radiograph (which may show atypical findings in HIV-infected patients) 1
- Microscopic examination and culture of sputum specimens (3-5 specimens collected on different days) 1
- Rapid molecular testing (Xpert MTB/RIF) when available 5
Common Pitfalls to Avoid
Never delay TST because a patient is coughing—cough is an indication for testing, not a contraindication. 1
Never rely solely on TST results to exclude active TB—proceed with full diagnostic workup including chest radiograph and sputum examination regardless of TST result. 1
Never assume a negative TST means the patient doesn't have TB, especially in immunocompromised patients where false-negatives are common. 1, 3
Never measure erythema instead of induration—only induration counts for interpretation. 4, 6
Never read the test outside the 48-72 hour window, as this reduces accuracy. 4