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Isoniazid (Serum) Concentration, Quantitative

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of isoniazid, an antibiotic used to treat tuberculosis, in your blood. It helps doctors check if the drug is working properly and adjust your treatment as needed.

Quick summary

["This test helps ensure your tuberculosis treatment is effective and safe by monitoring your drug levels."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
29315-9
Short code
Isoniazid Islt Slgr Mcbct AB 0.4
Synonyms
Isoniazid Islt Slgr Mcbct AB 0.4
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
method-for-slow-growing-mycobacteria-antibiotic-concentration-at-ug-ml
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your healthcare provider may order this test if you’re taking isoniazid for tuberculosis treatment. It helps determine whether your dose is appropriate or if you need adjustments to avoid side effects or prevent drug resistance.

What to Expect Before the Test

No special preparation is needed. You may have a small needle inserted into your arm to draw blood. If you’re on long-term treatment, your provider may schedule this test periodically to monitor your response.

What the Results Mean

Results are used to guide treatment decisions, such as adjusting your dose or switching medications if levels are too high or too low. Your provider will discuss these findings with you.

Common Reasons for Testing

This test is often done during tuberculosis treatment to ensure the drug is reaching effective levels in your bloodstream. It’s also used if you experience side effects or if the infection doesn’t improve as expected.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance: Monitoring Drug Levels for Safety and Effectiveness

Short name

Isoniazid Level

Frequently asked questions

How often should I get this test?
Your provider will determine the timing based on your treatment plan. Typically, it’s done during active treatment phases or if you’re experiencing symptoms like nausea, vomiting, or liver issues that may affect drug levels.
What happens if my isoniazid level is too high or too low?
High levels may cause side effects like liver damage or rash, while low levels might mean the drug isn’t working as intended. Your provider will adjust your treatment accordingly.
Can I drink alcohol before this test?
Alcohol can interfere with some medications, including isoniazid. Ask your provider about any restrictions before your test.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy