VN April 2023

Vetnuus | April 2023 36 Biochemistry analysis by organ system Part II Dr Coetzee de Beer The kidney The end product of protein metabolism in birds is uric acid . It is produced in the liver, enters the circulation, and is then secreted by renal tubules (>90%) or filtered in the glomerulus (< 10%). Significant loss of renal tubules will therefore see elevations of uric acid. Dehydration is less likely to cause hyperuricaemia because glomerular filtration is relatively unimportant. At first glance, it would appear that uric acid offers a sensitive and specific test for renal disease. There are, however, several confounding factors. Firstly, species differences; carnivorous birds have higher normal uric acid levels than granivorous birds. Secondly, age; juvenile birds may have lower levels than adults. Thirdly, although significant elevations usually indicate renal disease, normal levels do not mean the kidneys are normal; mild increases could indicate early renal disease or dehydration (or both). There must be severe renal damage before uric acid levels begin to rise. Because of this relative insensitivity of uric acid in detecting renal disease, levels are best interpreted alongside determining the bird’s water intake and loss and a physical examination. To distinguish renal disease from dehydration, the patient’s hematocrit, total protein and blood urea nitrogen (BUN) should be evaluated concurrently. Dehydration can lead to decreased glomerular filtration rates (GFR), in turn leading to elevated levels of BUN; this same decrease in GFR can lead to elevations of uric acid without primary renal disease being present. In cases of an elevated uric acid level, it is prudent to rehydrate the patient over 2-3 days before definitively diagnosing renal disease. Persistent hyperuricaemia after fluid therapy, and with haematocrit, total protein and BUN returning to normal, confirms a diagnosis of renal disease. Creatinine is generally accepted as being of little or no value in evaluating renal function in birds. Phosphorous elevations are usually not seen in birds with renal disease. The reproductive system Clinical biochemistries can tell the clinician little about the male reproductive tract; they can, however, reveal something about the activity of the female reproductive tract. Oestrogen, produced by developing follicles, induces the production of calcium-binding protein and vitellogenesis in the liver. The net result of this activity is an increase in circulating total protein, calcium , and cholesterol. The serum may appear lipaemic. Radiographic evidence of hepatomegaly and increased long bone density can confirm reproductive activity. It should be noted that normal calcium and protein do not reflect a lack of reproductive activity. The gastrointestinal system Gastrointestinal disease typically only gives non-specific results with clinical biochemistry. Elevations of CK, AST and LDH are not uncommon and are not specific to the intestinal tract. Electrolytes may give more information: Sodium may be elevated with excessive water loss through vomiting or diarrhoea; Chloride may be elevated with vomiting or regurgitation; and Potassium may be decreased with vomiting/diarrhoea and elevated with dehydration. Many other possible causes of electrolyte disturbance exist, and our understanding of avian electrolyte balance is still in the very early stages. Amylase and lipase have been proposed as useful parameters in detecting pancreatic disease. There is still considerable discussion of the incidence of pancreatic disease and the specificity of these enzymes. Significant elevations of these enzymes, when accompanied by clinical signs of gastrointestinal dysfunction (vomiting, ileus, diarrhoea, abdominal pain), should lead the clinician to consider pancreatic disease as a differential diagnosis. However, normal levels do not preclude a diagnosis of pancreatic disease, nor do abnormal levels confirm such a diagnosis. Technical I Article