Phenytoin test

What is Phenytoin?

Phenytoin is a drug that is used to control seizures and it is generally used as an anticonvulsant or antiepileptic drug. The drug is also known by other brand names like Dilantin or Eptoin. It works by slowing down the impulses in the brain and minimizing the activity of brain stem centers.

Phenytoin also acts as an anti-arrhythmic and a muscle relaxant, though the exact mechanism of its action is still not clear. While the drug has multiple therapeutic uses, its application is limited because of various adverse effects and interactions with other drugs.


Phenytoin is also used to prevent the occurrence of seizures during neurosurgery. The drug can result in adverse cardiovascular reactions and hence the rate of the administration needs to be controlled carefully. In general, intravenous administration in adults should not exceed 50 mg per minute.

Careful cardiac monitoring is needed during and after the administration of the drug to avoid any adverse cardiac reaction. Phenytoin is also available in oral forms and can be consumed as an extended-release oral capsule, chewable tablet or as an oral suspension.

It is important for the patient to follow the instructions of the doctor as written in the prescription in the exact form regarding the consumption of the medicine. In general, a physician starts with a low dose of phenytoin and gradually increases the dosage as needed.

What is the Phenytoin test?

The Phenytoin test determines the serum level of the drug so that the correct dosage can be determined. It is to be noted that the drug can only control a condition but cannot cure it.

What does the level of phenytoin indicate?

The serum levels of Phenytoin can vary depending on the patients’ medical history and condition. The high levels of the drug can be caused by liver disease, congenital enzyme deficiency or drug interactions resulting in metabolic interference. Since the drug is bound to proteins, patients with binding characteristics that differ from normal will have highly variable serum levels of the drug.

The use of the drug during pregnancy can cause serious birth defects. Hence the drug is generally avoided for a pregnant woman or a woman of childbearing age. It can also cause allergic reactions resulting in skin rashes, swollen lymph glands, and fever.

The drug can also decrease the vitamin D levels in the body along with lowering calcium and phosphate levels. It can also reduce the levels of the thyroid hormone and anyone with thyroid ailments should exercise precautions while consuming this drug.

Why do I need to undergo a Phenytoin test?

The test is generally conducted to check the blood level of the drug in order to administer the right dosage. The levels have to be controlled carefully as in case they are too low, seizures may occur. If they are too high, the person may experience symptoms associated with Phenytoin toxicity.

Once the correct levels that are apt for the patient have been reached, the test is done at frequent intervals to make sure that the levels are within the right range. The test is also ordered when the doctors suspect a condition resulting from Phenytoin toxicity. This may have various effects, some of which are listed below.

– Swelling of the gums and/or lymph nodes
– Excess facial and body hair (hirsutism)
– Insomnia
– Nausea and vomiting
– Alterations in sight, smell or taste
– Confusion and irritability
– Suicidal thoughts or behaviors along with panic attacks.
– Difficulty swallowing
– Fatigue
– Fever
– Rashes
– Involuntary eye movement(nystagmus)
– Abnormal heartbeat
– Low blood pressure
– Loss of balance or involuntary muscle convulsions.
– Osteoporosis or softening of bones.

How is a Phenytoin test performed?

The test is performed by a venipuncture collection of blood from a vein, usually by picking one from the arm. In this process, a needle is inserted into a vein to draw the required volume of blood. The process involves the following steps.

– An elastic band is tied in the upper arm to make the veins swell with blood, thus making them more visible. The collection site is generally around the elbow region and the technician uses alcohol to disinfect the area where the needle will be inserted. In some cases, blood may also be collected from the back of the hand.

– A needle is inserted into the vein and the required volume of blood is drawn through a syringe in an attached tube. A slight prick or stinging sensation is felt when the needle is inserted in the vein.

– The band is untied and the needle is withdrawn slowly after a gauze is placed on the site. The pressure is applied to the site through the gauze to prevent any excessive bleeding. The spot is then covered with a sterile bandage and the sample is sent to the laboratory for testing.

– The collection process is safe and is completed within a few minutes. Since the size of the veins can differ from one person to the other, multiple punctures may be needed for some patients before the right vein is found. Some patients might feel dizzy or lightheaded after the blood collection and may need proper care.

Any pain or mild bruising around the collection site is generally cured within a few days. Chances of infection or excessive bleeding from the site are rare but will need medical care in case they happen.

What is the price for a Phenytoin test in India?

The cost of the test ranges between Rs 500 to Rs 1800 depending on the place and facilities provided in the testing center.

When will I get the test results?

The test results are generally delivered within 24 to 72 hours.

What is the normal range for a Phenytoin test?

The Phenytoin levels can vary between 10 to 20 µg/ml for total Phenytoin, which includes bound and unbound levels, both in children and adults. This range is independent of the sex of the patient.  For new-borns, the levels can be between 8 to 15 μg/ml. Levels above 20 µg/ml are considered to be in the toxic range. This range is just a general guide and the levels are to be interpreted by a doctor for a correct diagnosis.

What do the results mean?

The test result depends on a variety of factors and needs to be interpreted by a doctor to relate it with any symptom. Just because the levels are high, it may not mean that there is a problem. Depending on the results and your condition, the doctor may adjust the dosage of Phenytoin as necessary. The dosage determinations and changes are carefully determined on a case-by-case basis. In addition, the doctor can also order some additional tests like the ones mentioned below.

– A complete blood count
– Blood urea nitrogen and creatinine test to check kidney function
– Liver function panel or other liver tests
– Glucose test to measure your blood sugar.
– Blood tests to measure the sodium levels.
– Tests for other medicine levels.
– Urinalysis

Do I need to fast for the Phenytoin test?

No fasting or any other special preparations are needed before the test.

What else do I need to know before I appear for a Phenytoin test?

The following points can be kept in mind before appearing for a Phenytoin test.

– Phenytoin is sometimes prescribed for other conditions such as trigeminal neuralgia, which causes sharp pain along the jawline and other causes of nerve pain. It is also used to control irregular heartbeat and the dosage is carefully monitored.

– Medicines like antiepileptic drugs, antidepressants, antiviral agents and gastric acid-reducing agents can affect the levels of Phenytoin.

– Discuss the details of all the medicines and supplements that you consume with your doctor before undertaking the test. Chronic alcohol consumption can also affect the levels.

– Since Phenytoin passes into breast milk, you should consult your doctor before breast-feeding. In case you conceive while consuming the medicine, you should immediately inform your doctor.

Unless the seizures are caused by a temporary condition you may need to consume Phenytoin every day for the rest of your life.

Can pregnant women undergo a Phenytoin test?

Pregnant women can undergo the test under medical supervision.

Is the Phenytoin test possible for new-born babies?

The test is possible for new-born babies as advised by the doctor.