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Bloodwork for Dogs on Phenobarbital

Bloodwork for dogs on phenobarbital is important considering the effects this drug has on a dog's body. While this drug is effective in managing seizures in dogs, as with many other drugs, it can cause potential side effects and its therefore important to routinely gauge how the dog's body is handling things. Many dog owners wonder how often this bloodwork is needed, and most of all, why such testing is needed in te first place. Veterinarian Dr. Ivana discusses how phenobarbital works, what effects it has on a dog's body and how dog owners can monitor things.

rash on a dog's belly

Phenobarbarbital for Dog Seizures

Phenobarbital is a medication of the barbiturate group that slows down the activity of the brain and nervous system. To be more accurate, phenobarbital works by controlling the nervous system’s abnormal electrical activity. Its most common uses are in the management of seizures and as short-acting sedative.

Although there are many newer medications, phenobarbital remains the treatment of choice for dogs with seizures. This is perhaps because phenobarbital is efficient regardless of the seizures’ underlying cause whether it is epilepsy, brain tumors, poisonings and infectious diseases. Additionally, phenobarbital is reasonably priced, conveniently dosed, and when used as instructed – relatively safe.

In most cases, phenobarbital is a life-long necessity. Dogs prescribed phenobarbital are likely to be obliged to receiving it for as long as they live. Skipping or missing as little as one dose can result in seizures.

Bloodwork For Dogs on Phenobarbital

As with any other long-term medication, dogs put on phenobarbital should be regularly checked and closely monitored. The best way of monitoring is through blood test. The monitoring blood tests should be performed periodically, but regularly. The most commonly used monitoring blood tests include: blood cell count, phenobarbital blood level and liver enzymes levels.

Blood Cell Count

Standard blood cell count should be performed in all dogs receiving phenobarbital. This is because in the first 6 months of treatment, this drug is likely to cause an overall decrease in the blood cell count. To be more precise, phenobarbital causes immune-mediated anemia (low red blood cells count), neutropenia (low neutrophils count – part of the white blood cells group) and thrombocytopenia (low platelets count).

Phenobarbital Blood Level Test

Phenobarbital is a potent medicine and to avoid overdosing it must be used carefully and as instructed. When the vet determines the right dose he considers several important factors. One of those factors is the level of phenobarbital in the patient’s blood.

All dogs receiving the drug phenobarbital should have their phenobarbital blood levels checked at least every 6 months. Generally speaking, the levels of phenobarbital in the blood should be monitored and evaluated in the following situations:

  • When initially prescribing the drug to determine its optimal dosage (to achieve effect without triggering side-effects)
  • When testing the efficacy of a new dose
  • If seizures develop while the dog is on therapy
  • If signs of liver disease develop while the dog receives this drug.

What does the test reveal? Simply put, the test reveals the levels of circulating phenobarbital in the dog’s blood. This is important because only a specific amount of this drug is considered therapeutic and safe. Therefore, if the test shows that the levels of phenobarbital are too high, the dose should be decreased.

Once the dose is reduced to safe levels, if seizures occur, the dog should be put on some additional medicines. On the other hand, if the test shows the levels of phenobarbital are too low, the drug’s dose should be increased until achieving efficacy. The effective therapeutic dose of phenobarbital for dogs is 65-150µmol/L.

How is a phenobarbital blood test for dogs performed? To perform this test, the vet will take a blood sample and store it in a clot-promoting glass tube. Once clot forms, the sample is centrifuged and two separate layers form – serum and blood clot. The blood clot is discarded and the serum is used for the analysis. It usually takes a day or two for the results to be ready.

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Is a phenobarbital blood test for dogs painful? Generally speaking, the phenobarbital blood level test is not painful. To collect the blood sample, a needle must pierce the skin and enter a blood vessel. Just like people, some dogs are not fans of needles. However, the procedure cannot be described as painful only as unpleasant.

Most dogs allow collecting blood without causing problems. However, in particularly aggressive dogs, sedation or ultra-short anesthesia is recommended. If the patient is too agitated, the collected sample may be too small for the analysis to be performed.

Liver Enzymes Level 

Phenobarbital acts as a booster that enhances and increases the activity of certain metabolic enzymes. When boosted, the enzymes become more potent and more efficient at removing toxins from the body. This boost is often manifested through elevated liver enzymes.

Therefore, in patients on phenobarbital, the liver enzyme levels should be regularly monitored. That way, any potential issues will be timely diagnosed and properly managed.

There many different monitoring protocols. However, in general, checking the liver enzymes is recommended following this timeline:

  • The first sample should be tested two weeks after initiating treatment
  • The second sample should be tested six weeks after initiating treatment
  • Then, blood samples should be tested every 6 months.

This timeline should be followed in patients that respond well. If the dog experiences a seizure while on medications or if there was a change in its drug’s dose, it is recommended to check the liver enzymes regardless of the above explained timeline. In cases of dose changes, the test should be performed two weeks after the change.

What does this test reveal? The liver enzyme test for dogs measures the levels of liver enzymes and the liver enzymes are indicative of the liver’s health status. Increased levels of liver enzymes can be indicative of hepatotoxicity (drug induced liver damage).

How is a liver enzyme test performed? The procedure is the same as in the previous test.

Is the test painful? Just like in the previous test, same patients find the blood drawing procedure to be quite unpleasant. On the other hand, some patients barely notice their blood is drawn.

Is sedation or anesthesia required? Most patients will allow blood drawing. However, agitated, aggressive or extremely stressed dogs should be lightly sedated prior to having their blood collected.

About the Author

Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia. She currently practices as a veterinarian in Bitola and is completing her postgraduate studies in the Pathology of Domestic Carnivores at the Faculty of Veterinary Medicine in Zagreb, Croatia.

ivana crnec

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