Clinical chemistry of calcium
Introduction
Calcium is an essential mineral which is involved in many body systems. These include the skeleton, enzyme activation, muscle metabolism, blood coagulation and osmoregulation. In the blood, calcium exists as 50% ionised, 40% protein-bound and 10% complexed with anions such as citrate and phosphate. Only ionised calcium is biologically active in bone formation, neuromuscular activity, cellular biochemical processes and blood coagulation. Factors governing the total plasma concentration are complex and include interaction with other chemical moieties, proteins and hormones. Calcium, phosphorus and albumin metabolism are interdependent. In the dog serum calcium values may be adjusted to take account of low albumin levels.
Small animals
Homeostatic mechanisms in small animals result in tight control of plasma calcium levels. Elevated and decreased levels are generally significant.
Causes of hypercalcaemia
- Primary hyperparathyroidism (primary parathyroid hyperplasia/neoplasia)
- Hypoadrenocorticism
- Cholecalciferol rodenticides
- Primary renal failure (CRF, ARF, familial)
- Hypercalcaemia of malignancy (malignant tumours including lymphoid tumours, myeloproliferative disease and some carcinomas) producing parathyroid like hormone
- Granulomatous disease
Causes of severe hypocalcaemia
- Eclampsia
- Primary hypoparathyroidism
- Ethylene glycol poisoning
Causes of mild to moderate hypocalcaemia
- Hypoalbuminaemia
- Acute pancreatitis
- Renal secondary hyperparathyroidism
- Nutritional secondary hyperparathyroidism
- Acute renal failure (especially post renal obstruction)
- Post thyroidectomy (cats)
Complementary tests
PTH assay, serum electrolytes and ACTH stimulation test, urea and creatinine, lipase, albumin and phosphorus. See section on calcium disorders.
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Equine
Homeostatic mechanisms are efficient in maintaining tight control of plasma calcium levels. Elevated and decreased levels are therefore significant. Slight increases may occur in hot weather and decreases following increased carbohydrate utilisation.
Causes of hypercalcaemia
- Renal failure (chronic)
- Intravenous calcium
- Hypervitaminosis D – supplementation
Causes of hypocalcaemia
- Hypoalbuminaemia
- Enterocolitis in adults
- Sepsis (foals)
- Lactation and transport tetany
- Acute renal failure
- Hypoparathyroidism
Consequences of hypocalcaemia
- Synchronous diaphragmatic flutter in performance horses
- Uterine inertia in full term pregnant mares
Complementary tests
Albumin and phosphorous determinations should always be included. Differentiation of diseases causing abnormal calcium levels can be facilitated by the determination of fractional clearance of calcium in blood and urine.