Investigation

Serum creatinine & urea
Production of creatinine principally comes from metabolism of skeletal muscles. Since the level varies a little throughout the day, 24-hour urine collection is needed to reduce errors. The variations are due to changes in hydration or posture, tubular creatinine secretion etc. It is freely filtered by glomerulus, not metabolised, little or none is reabsorbed while small amount is secreted. These properties mean the amount of creatinine filtered almost equals to that excreted. Also, it is important to note that creatinine level is related to age, sex and muscle mass. Normal range is between 0.8-1.5mg/dL (70-135μmol/L).

Urine creatinine level is also taken to calculate glomerulus filtration rate (GFR):
U x V/P where U = urine concentration of creatinine; V = urine flow (mL/min); P = plasma concentration of creatinine. Normal rages are 90-140 and 80-125 mL/min for men and women respectively.

Renal failure results in reduced excretion of nitrogenous waste, e.g. urea, in which the serum concentration would be raised. Normal range lies between 2.5 - 7.5 mmol/L.

In renal diseases, a reduced renal blood flow, damage to or loss of glomeruli, or obstructed ultrafiltration along the tubules reduce GFR. Creatinine and urea are retained and give a raised serum level. However, both serum urea and creatinine do not rise above the normal range until there is a reduction of 50-60% in GFR.

Serum potassium
One of the functions of kidney is the body regulation of electrolytes e.g potassium. When kidney function is impaired, there is a reduced reabsorption of potassium, thus increases serum potassium concentration. Hyperkalaemia particularly increases the risk of cardiac dysrhythmia. Normal level is about 190mg/L.

Renal ultrasound
This can be used to differentiate between chronic and acute renal failure, in which the former usually results in a smaller kidney. Likewise, the images confirm the presence of any bladder outflow obstruction, urethral or ureteric stricture or obstruction etc. – suggesting post-renal cause of renal failure.