Definition

Therapeutic drug monitoring is a test that identifies the concentration of specific medications in the bloodstream. This test measures the amount of phenobarbital in your blood. If your healthcare practitioner prescribes phenobarbital for your medical conditions, you may require this test. It is done to ensure that the dosage of medication you are taking is safe and efficient. 

Phenobarbital, classified as a barbiturate, acts as a sedative-hypnotic medicine. Its function is to slow brain activity. Phenobarbital has a diverse range of clinical applications and is often used for managing seizures. Doctors also suggest it as a treatment for status epilepticus, a medical emergency characterized by continuous seizure episodes for 5 minutes or more. Phenobarbital, while beneficial for this condition, has mostly been substituted with alternative medications with fewer harmful adverse effects.

Ensuring whether the treatment is effective without triggering harmful side effects can be challenging. When phenobarbital blood levels rise above a certain threshold, the patient is at a higher risk of experiencing phenobarbital side effects.

We also have an article on seizure which you can read here: Seizure - Definition, Risk Factors and Management

Indication

When taking phenobarbital to manage seizures, it is crucial to maintain the appropriate level of the medication in your bloodstream. This level is referred to as the therapeutic range. Your therapeutic range is the optimal dosage of phenobarbital needed to effectively manage your seizures while minimizing unwanted effects. 

This test determines if your phenobarbital dosage falls within the commonly acknowledged therapeutic range for the medication. Having a phenobarbital level in the blood below the normal limit may increase the likelihood of experiencing a seizure. Exceeding the high amount may increase the likelihood of experiencing negative effects.

You may have this test:

• Several days to a few weeks after taking the medication
• Following any modifications to the medication dosage
• After adding another anticonvulsant to your therapy regimen
• If your healthcare provider prescribes other drugs that may interact with phenobarbital
• If seizures persist despite phenobarbital treatment
• If you become pregnant 

If you experience adverse reactions to phenobarbital or if your doctor suspects elevated levels of this medicine in your body, this blood test is necessary.

Contraindication

There are no individuals for whom it is advised against to check their serum phenobarbital level; everyone is eligible to undergo blood tests for monitoring the levels of phenobarbital in their bodies.

Preparations Prior to Test

There is no need to prepare for this test. Ensure that your healthcare provider is aware of all medications, vitamins, supplements or herbs you are currently taking. This encompasses over-the-counter medications and any illicit substances you may consume. Various frequently prescribed medications, such as birth control pills, can have an impact on phenobarbital and in turn, the test results.

Inform your healthcare practitioner about the amount of phenobarbital you have taken. Seek advice from your doctor about whether you should take your usual medications before the test or not.

Test Procedures

A healthcare professional will extract a blood sample from a vein, usually in your arm, using a tiny needle. Once the needle is placed, a little sample of blood will be drawn into a test tube. You may experience a slight stinging sensation when the needle is inserted or removed.

Normal and Abnormal Values

Your results will indicate whether the amounts of phenobarbital in your blood fall within a therapeutically beneficial yet non-hazardous range. Phenobarbital is measured in micrograms per milliliter (mcg/mL). 

The range of phenobarbital deemed effective without causing issues for adults is between 10 to 40 mcg/mL. Once blood levels exceed 40 mcg/mL, the patient is at significant risk to experience harmful adverse effects of phenobarbital.

Results and Suggestions (Follow-Up Tests)

If your test results fall outside of this therapeutic range, your healthcare professional may need to modify your phenobarbital doses. For example, if your blood level is elevated, your phenobarbital dose may need to be reduced. If your blood level is too low, you may require a higher phenobarbital dosage. 

Barbiturate toxicity becomes apparent at a dosage of 1 gram when taken orally, but this can vary from person to person. Doses exceeding 2 grams can be fatal, with the lethal dose often falling within the range of 40 to 80 mcg/mL.

Increased phenobarbital concentrations in the bloodstream enhance the risk of toxicity. Barbiturate toxicity can vary, but typical symptoms include:

• Nausea
• Muscle weakness
• Excessive thirst
• Cognitive impairment
• Decreased heart rate
• Lack of coordination
• Decreased urine production
• Low body temperature
• Pupil dilation (midriasis)
• Fatalities have occurred due to severe respiratory depression, low blood pressure, and coma

After your dosage is modified, you may need to undergo several tests until your phenobarbital levels are within the ideal range. Your doctor will analyze your blood test results, in conjunction with other variables, to determine if any adjustments are necessary to your therapy.

Consult To The Right Doctor

Test results can differ based on factors such as age, gender, medical history, or other factors such as the utilized laboratory. Consult your doctor or neurologist to interpret the implications of your test results.

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