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CARBAMAZEPINE

About the test
What is this test done for?
Carbamazepine is a blood test done for checking the amount of drug carbamazepine in blood.
Why is it Done?
This is a specialized test done for assessing the amount of carbamazepine in blood as the drug is used as a first line of medication for generalized and partial complex seizures (epilepsy/convulsions). Prolonged use or overdose can cause toxicity to the body. Thus this test is performed for adjusting the dose..
When should it be performed?

This test is performed when a patient on carbamazepine presents with symptoms of its toxicity, which includes

  • Mild to moderate overdose
    • Nystagmus (involuntary eye movements)
    • Ataxia (lack of voluntary co-ordination of muscles)
    • Ophthalmoplegia (Paralysis of muscles within or surrounding eyes)
    • Dyskinesia (involuntary movements)
    • Dystonia (Uncontrolled involuntary contraction of any single or group of muscles)
    • Mydriasis (dilation of pupil of the eye)
  • Severe toxicity
    • Seizures or convulsions
    • Respiratory depression
    • Coma
    • Tachycardia (High pulse rate)

Long term use of carbamazepine causes hepatitis, bone marrow depression (low WBC count, low hemoglobin), kidney failure, dermatitis (skin problems) etc.

How is it done
A blood sample will be collected from a vein in your arm. Fasting is not needed. The blood is collected just before the next carbamazepine dose.
Results

The test is negative if levels are < 12 mg/L. It means the dosage of carbamazepine is appropriate for the patient.

The test is positive if levels are > 12 mg/L and this means one of the following-

  • High dose of carbamazepine
  • Overdose of carbamazepine

Toxicity or Critical levels are > 40 mg/L. This means either

  • Severe toxicity of carbamazepine
  • If not given antidote, may result in death
Other Tests
  • HLA-B 1502
    • This test is done (usually in Asian patients) before starting the carbamazepine therapy
    • Positive test indicate increased risk of toxic/allergic reactions
  • HLA-A 3101
    • This test is done (usually in European patients) before starting therapy
    • Positive test indicate higher risk of toxic skin reaction (Stevens-Johnson Syndrome) to carbamazepine
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