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ASK THE VET (other parameters that are evaluated on a blood chemistry panel)

by Linda Pesek, DVM
Westbury Animal Hospital
319 Union Avenue
Westbury, NY
516-333-1123

This article first appeared in SQUAWK, the newsletter of the Big Apple Bird Association and is reprinted with permission. This article is one in a continuing series which describes diagnostic tests performed by the avian vet.

In this column, I would like to finish discussing some of the other parameters that are evaluated on a blood chemistry panel.

Liver Function:

One way liver disease may be manifested in dogs and cats is by the deposition of bilirubin in the skin, mucus membranes and sclera (or whites) of the eyes. Bilirubin is formed in the liver from the breakdown of aged red blood cells. When excess bilirubin is present as a result of the diseased liver's inability to handle it, it becomes deposited in the tissues, leading to a yellowing known as icterus or jaundice. Birds lack the enzyme necessary to form bilirubin and thus form biliverdin instead. Biliverdinuria, or the deposition of greenish pigments in the urine, occurs instead of jaundice in birds with liver disease.

In the past it has been very difficult to evaluate liver function in birds. The enzymes traditionally used were not liver specific and became elevated for many different reasons in addition to liver disease. A fairly new test used to evaluate liver function in birds (as well as dogs and cats) is bile acids. The advantage is that is is liver specific and therefore is a very good indicator of liver function. Bile acids are formed in the liver from cholesterol and are excreted in the intestine where they assist in the digestion of lipids.

Creatine Phosphokinase:

Creatine Phosphokinase is an important enzyme that may be elevated in muscle damage. It will also be increased with physical exercise, neuropathies, lead poisoning, psittacosis, and certain irritating antibiotics given by injection.

Calcium:

Normal blood calcium ranges from 8-18 milligrams per decaliter (mg/dl). Hypercalcemia, or elevated calcium levels, can occur with excess vitamin D3 supplementation, or as a normal physiological occurance in egg laying hens. Hypocalcemia, or very low calcium, can occur with an all seed diet, or renal failure. Seizures can occur when calcium falls below 6!

Phosporus:

Normal phosphorus levels range from 2 - 4.5 mg/dl. Hyperphosphatemia occurs in renal disease, or diets with excess vitamin D3. Hypophosphatemia occurs in starvation, loss of appetite, or intestinal disease.

[NB: Dietary calcium:phosphorus ratio should be 1.5 - 2.5 :1 with calcium present in greater amounts than phosphorus. An all seed diet can range from 1(CA):10(Phos) in parrot seed to 1(CA):37(Phos) in parakeet seed, with phosphorus present in larger amounts.]

Uric acid (as an indicator of renal function):

Birds are uricotelic which means they produce uric acid as a major end product of amino acid breakdown. (Note: mammals are ureotelic, producing urea as a major end product of amino acid breakdown.) Uric acid is relative nontoxic when compared to urea or ammonia, the major products of amino acid breakdown in mammals. Uricotelism allows excretion or storage of nitrogenous waste in a small amount of water. This (and its low toxicity) is important for the development of the embryo in the egg of the bird. Uric acid is synthesized in the liver and most of it is excreted by tubular secretion in the kidneys.

Uric acid levels increase when renal function has been reduced to 30% of its original capacity. Hyperurinemia is a terms used to indicate elevated uric acid in plasma. Normal uric acid levels range from 2-15 mg/dl. Levels greater than 20 are considered elevated. In addition to renal disease, a number of other factors can elevate uric acid. These include: massive tissue destruction, starvation, gout, calcium deposition in the kidneys due to a diet too high in calcium or vitamin D3, and an excess of protein in the diet. In addition, a blood sample taken from a toe nail may be contaminated with urates from droppings which will falsely elevate uric acid.

Urea is one of the parameters used to assess renal function in dogs and cats. Since birds are uricotelic, producing uric acid rather than urea as a result of amino acid breakdown, it is not considered a useful test of renal function in birds. Urea levels, however, will be elevated in shock and severe dehydration. These conditions can lead to renal failure if not corrected.