If risk factors or symptoms are present, TB testing is strongly advised.
TB Test(s) of Choice
In general, patients should receive a QuantiFERON test (QFT) unless:
- Phlebotomy is refused
- Phlebotomy is impractical (e.g., no veins, very young child)
- The specimens cannot be transported to the SFDPH laboratory before 4:30 PM Monday-Friday and the patient cannot return
for phlebotomy during the specified hours
- QFT test is not routinely available
If patient refuses test:
- Patient may opt for the TB skin test (TST) instead and will need to return to clinic in 48-72 hours following placement for the
skin test reading.
For special circumstances additional tests are needed to maximize the sensitivity of diagnosing LTBI:
- TB suspects (ATS class TB 5) with no prior testing or past negative TB test results:
- Use both the TST and QFT to increase sensitivity (may be done on the same day)
- Collect 3 sputum for AFB examination and culture
- Obtain a chest x-ray if most recent film was taken prior to symptom development or is older than 3 months
- Asymptomatic patients who are TB test negative and on immunosuppressive therapy:
- Use QFT or TST as a second test
- Obtain a chest x-ray
- Immunosuppressive agents likely to cause false negative results include prednisone, methotrexate, post-transplant immunosuppressive medications and other cancer chemotherapeutic agents
- The chest x-ray shall serve as a third diagnostic to look for evidence of TB infection (granuloma, hilar calcification, apical pleural thickening, upper lobe volume loss, fibrotic infiltrate(s), etc.)
TST Positive Criteria
When using the Mantoux method, the skin test should be evaluated within 48-72 hours of administration. Induration (palpable swelling), not erythema (redness) should be measured. The result should be documented and recorded in millimeters of induration and entered into the medical record and electronic database.
The skin test is considered positive if the reaction is:
- ≥ 10 mm OR
- ≥ 5 mm AND the patient is
- A close contact to an infectious TB case
- A person with radiographic evidence of old, healed TB
Multiple-puncture tests should not be used because the results are less reliable.
The TST and QFT are only diagnostic aids, and a negative result cannot rule out active TB. Persons with active TB disease may have a negative skin test due to overwhelming infection or anergy. Clinical judgment is always needed in relation to symptoms, presentation and risk factors when interpreting results.
Test Converter Definitions
- Not a TB contact: Current positive test with a documented prior negative result within the past two years AND a ≥ 0.75 IU/ml
quantitative increase from prior result
- Recent contact to known active case: Current positive test with a documented prior negative result within the past two years
TST converter: Recorded negative TST less than two years prior to a positive TST AND an increase of ≥10 millimeters induration from prior result.