Laboratory tests of renal function

The kidneys are vital organs in the management of fluid balance, waste product removal, electrolyte homeostasis, acid–base balance, and endocrine function. Waste products removed by the kidney are urea, uric acid and creatinine; other foreign products with similar physio-chemical properties are also excreted. Urea and uric acid are by-products of protein metabolism and creatinine is generated by the metabolism of creatine compounds from muscle. The kidney regulates fluid and electrolyte balance through controlling the composition and volume of urine. In the proximal convoluted tubule and the loop of Henle, 90% of sodium, potassium, calcium and magnesium are reabsorbed. Acid–base balance is achieved by regulating the excretion of hydrogen ions and bicarbonate buffering. The kidney also has several endocrine functions including the production of renin and erythropoietin as well as hydroxylation of vitamin D. The kidneys receive 25% of cardiac output, generating 170–200 litres of ultrafiltrate daily. Urine output is approximately 1.5 litres/day, which is concentrated ultrafiltrate through selective reabsorption of solutes and water. In this article we will discuss tests frequently used to assess renal function.