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Isoniazid Concentration in Serum (Quantitative Drug Level)

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 determine how well your body is responding to the medication.

Quick summary

["This test helps ensure your tuberculosis treatment is effective and safe."]

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

Key facts at a glance

LOINC
20383-6
Short code
Isoniazid Islt SlowMyco
Synonyms
Isoniazid Islt SlowMyco
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
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 Is This Test Done?

Your healthcare provider may order this test if you’re taking isoniazid to treat tuberculosis or prevent it in high-risk situations. It’s often checked during treatment to monitor drug levels and adjust dosing if needed.

How Is the Sample Collected?

A blood sample is drawn from a vein, usually in your arm. You may feel a small pinch when the needle is inserted, but it usually goes away quickly. The sample is then sent to a lab for testing.

What Does the Test Result Mean?

Results are compared to a general range of expected values, but exact numbers can vary by lab. Low levels may indicate the medication isn’t working well, while very high levels could signal side effects or improper dosing. Your doctor will interpret the result in the context of your treatment plan.

What Are Common Reasons for This Test?

Doctors may order this test to: check if isoniazid is working to treat tuberculosis, monitor for side effects like liver damage, adjust your dose if needed, or confirm you’re taking the medication as prescribed.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Isoniazid Level

Frequently asked questions

How long does it take to get results from this test?
Results are typically available within 1 to 3 business days, depending on your lab’s processing time. Your doctor will contact you once they have the results.
Can I eat or drink before this test?
You can eat and drink normally before this test, as it only involves a blood draw. However, always follow any specific instructions your doctor provides.
What should I do if I feel dizzy or lightheaded after the blood draw?
If you experience dizziness or lightheadedness, lie down for a few minutes and rest. Apply gentle pressure to the site where the needle was inserted to help stop any bleeding. Contact your doctor or seek medical attention if symptoms worsen.
  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