What is the management for an indeterminate Tuberculin Gold (TB Gold) result?

Management of Indeterminate TB Gold Test Results

For patients with indeterminate QuantiFERON-TB Gold (QFT-G) test results, the optimal management approach is to repeat QFT-G testing with a newly obtained blood specimen or administer a tuberculin skin test (TST), with the decision based on the patient's risk factors for TB infection. 1

Understanding Indeterminate Results

Indeterminate QFT-G results provide no useful information regarding the likelihood of Mycobacterium tuberculosis infection. These results can occur due to:

• High background levels of interferon-gamma in the nil sample
• Inadequate response to mitogen (often associated with immunosuppression) 1, 2

Management Algorithm

Step 1: Identify Risk Factors for TB Infection

• Recent contact with infectious TB cases
• Birth or residence in high TB-incidence countries
• Immunocompromised status
• Abnormal chest radiograph consistent with prior TB
• Age (very young or elderly patients have higher rates of indeterminate results) 3

Step 2: Determine Management Based on Risk Assessment

For High-Risk Individuals:

• Repeat testing is recommended - either repeat QFT-G or administer TST 1, 2
• Consider chest radiography to rule out active TB disease
• If clinical suspicion for TB disease exists, proceed with additional diagnostic evaluations without delay:
  • Chest radiography
  • Bacteriologic studies
  • HIV testing 1

For Low-Risk Individuals:

• No further testing is necessary after an indeterminate QFT-G result 1
• Clinical monitoring may be appropriate

Step 3: Interpret Repeat Test Results

• If repeat QFT-G is positive: Evaluate for TB disease before diagnosing latent TB infection (LTBI)
• If repeat QFT-G is negative: No further evaluation needed in most cases
• If repeat QFT-G remains indeterminate: Consider the pattern of indeterminate results
  • Persistently indeterminate results are often of the same type (low mitogen or high nil) 3

Special Considerations

Immunocompromised Patients

• Indeterminate results due to inadequate mitogen response are more common in immunosuppressed individuals 2
• Factors associated with indeterminate results include:
  • Corticosteroid use (adjusted OR 2.92) 4
  • Low albumin levels <3.5 g/dL (adjusted OR 3.62) 4
  • Elevated inflammatory markers (CRP, ESR) 5, 4
  • Advanced age 5

Hematological Factors

• Abnormal white blood cell counts and lymphocyte percentages are predictors of indeterminate results in patients with hematological malignancies 6
• Consider timing of QFT-G testing in relation to immunosuppressive therapy

Pitfalls to Avoid

• Don't delay diagnostic evaluation for suspected TB disease while awaiting QFT-G results 1
• Don't assume an indeterminate result rules out TB infection - it provides no useful information regarding infection status 1, 2
• Don't ignore laboratory reports on the reason for indeterminate results (high nil vs. low mitogen) as this may guide clinical decision-making 1
• Don't overlook technical factors - improper handling or processing of blood specimens can cause indeterminate results (blood must be incubated with test antigens within 12 hours of collection) 2

By following this structured approach to managing indeterminate QFT-G results, clinicians can make appropriate decisions regarding further testing and evaluation based on individual patient risk factors for TB infection.