How is a TB (tuberculosis) skin test reading interpreted in a patient with a history of HIV/AIDS (human immunodeficiency virus/acquired immunodeficiency syndrome) or other immunocompromised conditions?

TB Skin Test Interpretation in HIV/AIDS and Immunocompromised Patients

Direct Answer

In HIV-infected and other immunocompromised patients, a tuberculin skin test (TST) induration of ≥5 mm is considered positive and indicates M. tuberculosis infection requiring evaluation for active TB and treatment for latent TB infection if active disease is excluded. 1, 2, 3

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Risk-Stratified Interpretation Cutoffs

The interpretation of TST results depends critically on the patient's risk category, with immunocompromised patients requiring the lowest threshold for positivity:

High-Risk Patients (≥5 mm = Positive)

• HIV-infected persons require a ≥5 mm induration cutoff regardless of CD4 count 1, 2, 3
• Recent close contacts of persons with active pulmonary or laryngeal TB 1, 3
• Organ transplant recipients and other immunosuppressed patients receiving the equivalent of >15 mg/day of prednisone for ≥1 month 1
• Persons with fibrotic changes on chest radiograph consistent with prior TB 3

Moderate-Risk Patients (≥10 mm = Positive)

• Injection drug users 3
• Residents and employees of high-risk congregate settings (jails, nursing homes, hospitals, homeless shelters) 1
• Persons with medical conditions increasing TB risk (silicosis, diabetes mellitus, chronic renal failure, hematologic disorders, head/neck/lung malignancies, gastrectomy, jejunoileal bypass, weight loss >10% ideal body weight) 1
• Children <4 years of age 1

Low-Risk Patients (≥15 mm = Positive)

• Persons with no known risk factors for TB 1, 3

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Critical Considerations for HIV/AIDS Patients

Anergy Testing Is NOT Recommended

Anergy testing is not recommended for routine use in HIV-infected or other immunocompromised persons. 1 The CDC explicitly revised this recommendation due to:

• Lack of standardization in anergy testing methods and antigens 1
• Poor reproducibility with unpredictable serial test results 1
• No documented benefit in screening programs for M. tuberculosis infection 1
• Selective nonreactivity to PPD can occur even in culture-positive TB patients while maintaining response to other antigens 1

High False-Negative Rate

• HIV-infected patients have a 61% rate of negative TST even with active TB disease 4
• Patients on immunosuppressant therapy show 50% negative TST rates 4
• The TST has a reported 25% false-negative rate during initial evaluation of active TB 3

Serial Testing May Improve Sensitivity

• TSTs may perform better with serial testing than interferon-gamma release assays (IGRAs) in HIV-positive patients 5
• LTBI screening should be performed at HIV diagnosis, when CD4 count rises above 200, and yearly with repeated exposure 5

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Proper Test Administration and Reading

Timing

• Read between 48-72 hours after injection when induration reaches maximum size 1, 2, 3
• Tests read after 72 hours underestimate true induration size 3
• For contacts of infectious TB, a negative test <8 weeks after exposure is unreliable; repeat at 8-10 weeks after exposure ends 2

Measurement Technique

• Measure only induration (hardened area), NOT erythema (redness) 1, 2, 3, 6
• Measure the transverse diameter perpendicular to the long axis of the forearm 3
• Record result in millimeters, even if zero 3
• Trained healthcare personnel must perform the reading—patient self-reading is unacceptable 2, 3

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Mandatory Follow-Up Actions

For Any Positive TST (≥5 mm in HIV/Immunocompromised)

1. Obtain chest radiograph (posterior-anterior view for adults; both PA and lateral for children <5 years) to exclude active pulmonary TB 1, 2
2. Perform clinical evaluation for TB symptoms (cough, fever, night sweats, weight loss) 1
3. Obtain sputum samples if symptoms present or radiograph abnormal, even if chest X-ray is negative in HIV patients 5

If Active TB Is Excluded

• Initiate preventive therapy with isoniazid for 9-12 months 2
• This is particularly critical for HIV-infected individuals given their substantially increased risk of disease progression 1, 5

Special Chest Radiograph Indications

The following contacts should have chest radiographs regardless of TST result: 1

• Persons with TB symptoms
• Immunosuppressed persons or those with risk factors for progression
• Children <5 years of age

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Common Pitfalls to Avoid

• Do NOT dismiss positive TST in BCG-vaccinated patients—tuberculin skin testing is not contraindicated in BCG recipients, and positive reactions should be interpreted as M. tuberculosis infection when the person is at increased risk 1, 3
• Do NOT rely on anergy testing to guide treatment decisions in HIV/immunocompromised patients 1
• Do NOT measure or record erythema alone—only induration counts 2, 3, 6
• Do NOT accept patient self-reading of results 2, 3
• Do NOT delay reading beyond 72 hours—this reduces accuracy 2, 3
• Do NOT use TST or IGRA to diagnose active TB disease—these tests are often negative with active disease 5

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Algorithm for HIV/Immunocompromised Patients

1. Administer TST using Mantoux technique (0.1 mL of 5 TU PPD intracutaneously) 2
2. Read at 48-72 hours by trained personnel, measuring only induration 2, 3
3. If ≥5 mm induration:
   • Obtain chest radiograph 1, 2
   • Evaluate for TB symptoms 1
   • If symptoms or abnormal radiograph: obtain sputum for AFB smear, culture, and nucleic acid amplification 1, 5
   • If active TB excluded: initiate isoniazid preventive therapy 2
4. If <5 mm induration:
   • Consider repeat testing at 8-10 weeks if recent exposure 2
   • Consider LTBI treatment based on exposure history and clinical judgment, particularly in HIV patients with known TB contact 1, 5