What laboratory tests should be monitored when prescribing isoniazide (Isoniazid)?

Laboratory Monitoring for Isoniazid Therapy

Liver function tests (LFTs) should be monitored regularly in patients taking isoniazid, with baseline testing required for all high-risk patients and monthly monitoring for those with abnormal baseline results or risk factors for hepatotoxicity. 1, 2

Baseline Laboratory Testing

Baseline liver function tests are indicated for the following patients:

• Patients with suspected or known liver disease
• Patients over age 35
• HIV-infected individuals
• Pregnant women and women within 3 months postpartum
• Individuals with history of chronic liver disease (hepatitis B or C, alcoholic hepatitis, cirrhosis)
• Regular alcohol users
• Patients taking other potentially hepatotoxic medications
• Patients with abnormal initial clinical evaluation 1, 2

Specific tests to obtain at baseline:

• AST (SGOT)
• ALT (SGPT)
• Bilirubin 1, 2

Monitoring Schedule During Treatment

For High-Risk Patients:

• Monthly liver function tests for:
  • Patients with abnormal baseline LFTs
  • Patients with risk factors for liver disease
  • Patients over age 35
  • Regular alcohol users
  • Pregnant women and those in postpartum period
  • HIV-infected individuals 2

For Standard Patients:

• Clinical evaluation at least monthly
• Laboratory testing only if symptoms develop 1

Special Monitoring for Specific Patient Groups:

• For patients with liver disease, alcohol abuse, or hepatitis B coinfection: monitor LFTs more closely 1
• For patients on concurrent DMARDs or biologics: more frequent LFT monitoring is advised due to higher risk of abnormalities (24% vs 12% baseline) 3

Criteria for Medication Discontinuation

Stop isoniazid if:

• Transaminase levels exceed 3× upper limit of normal with symptoms
• Transaminase levels exceed 5× upper limit of normal without symptoms 1, 2
• Development of symptoms of hepatotoxicity (unexplained anorexia, nausea, vomiting, dark urine, jaundice, fatigue, right upper quadrant pain) 2

Additional Monitoring Considerations

Drug Level Monitoring

• Not routinely required
• Consider only for:
  • Known or suspected malabsorption
  • Poor treatment response
• Target level: 3-5 mg/mL (peak)
• Timing: 2 hours post-dose 1

Adherence Monitoring

• INH test strips can be used to measure adherence
• BBL Taxo INH Test Strips detect isonicotinic acid (isoniazid metabolite) 1

Important Caveats and Pitfalls

1. Age-related risk: Hepatotoxicity risk increases with age, with rates of 4.4/1000 for ages 25-34,8.54/1000 for ages 35-49, and 20.83/1000 for those ≥50 years 4

2. Asymptomatic hepatotoxicity: Most cases of moderate-to-severe hepatotoxicity occur without symptoms, highlighting the importance of laboratory monitoring in high-risk groups 4

3. Baseline abnormalities: Abnormal baseline LFTs are a stronger predictor of developing hepatotoxicity than age 5

4. Early onset hepatotoxicity: Most severe reactions occur within the first 3 months of treatment, with highest risk between 1-3 months 6, 4

5. Drug interactions: Monitor more closely when isoniazid is combined with other hepatotoxic drugs, as rifampicin may enhance isoniazid hepatotoxicity 6

By following these monitoring guidelines, the risk of severe hepatotoxicity can be minimized while maintaining effective treatment for tuberculosis infection.