Phenobarbital Test

This test is done to monitor the levels of phenobarbital in the blood. Your doctor may order this test in the following conditions:

• A few days after starting the drug
• If the dose is modified
• If another anti-epileptic medicine is added
• If a drug that may interact with phenobarbital is initiated
• If the seizures continue while you are still taking phenobarbital 
• If you get pregnant

This test may also be ordered if there is a change in your therapeutic range over time:

• At the beginning of the therapy, you may be started on a higher dose.
• Over time, the doctor may increase your dose to maintain the same effect.
• Over the age of 70, the dose may need to be reduced.

Additionally, if you experience side effects or toxic symptoms from phenobarbitals or if the doctor suspects your phenobarbital level is too high, he/she may order this test. 

The side effects of phenobarbital include:

• Sleepiness
• Fatigue
• Depression
• Dizziness
• Decreased libido
• Memory problems
• Impotence
• Slurred speech
• Impaired attention
• Nausea
• Anaemia
• Rash
• Fever
• Decreased calcium levels 
• Folate deficiency

Symptoms of phenobarbital toxicity are:

• Drowsiness
• Slow breathing
• Decreased body temperature
• Loss of coordination
• Blisters
• Nystagmus (uncontrollable eye movement)

You do not need any preparation for this test. Tell your doctor if you are taking any other drugs as phenobarbital can have interactions with a lot of commonly used drugs, such as:

• Oral contraceptives
• Blood thinners like warfarin
• Disulfiram 
• Doxycycline
• Griseofulvin
• Hormone replacement therapy
• Medicines for cold, allergies, asthma, pain, depression or anxiety
• Monoamine oxidase inhibitors such as isocarboxazid, selegiline and tranylcypromine
• Other antiepileptic drugs such as phenytoin and valproate
• Corticosteroids such as methylprednisolone and prednisone
• Tranquillizers
• Sedatives

Also, inform your healthcare provider if you consume alcohol, as it may affect the test results.

(Read more: Effects of alcohol on body)

A phenobarbital test is performed on a blood sample. A laboratory technician will draw a small amount of blood from a vein in your arm. You may feel a mild pricking sensation when the needle is pierced, but it will subside in some time.

Some people get temporary bruising around the needle insertion site after the test - it subsides on its own. If you notice persistent discomfort please talk to your doctor.

Results of this test are measured in micrograms per millilitre (mcg/mL). However, the results would vary depending on the age, gender and health of the individual.

Normal results:

For infants aged 0-2 months, normal phenobarbital levels fall in the range of 15 and 30 mcg/mL. Phenobarbitals above 40 mcg/mL or more are toxic.

For children aged three months and above and adults, the normal range of phenobarbital is 15-40 mcg/mL, and the toxic levels are 50 mcg/mL or more. 

Phenobarbital levels should ideally be in the normal range but may be slightly above or below the range as prescribed by the doctor depending on how you respond to the treatment.

Abnormal results:

If your results are too high, your doctor may lower your dose to prevent toxicity. If your results are too low, the doctor may increase your dose if you are not responding to the therapy appropriately. Your doctor will make a decision on any changes to your dosage depending on your test results and response to treatment.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.