The human kidneys filter approx. 1500 liters of blood per day to remove toxins and metabolites such as urea from the blood. The filtration process also regulates concentrations of electrolytes and acidity. Reabsorption of some small molecules, inorganic ions, and water results in an average urine output of 0.5 to 2 liter per day.
Kidney disease has many causes (e.g. diabetes) but often no symptoms in its early stages. It can proceed unnoticed until kidney function is significantly impaired. The occurrence of kidney disease is common, disease progression is harmful, but nephropathies are well treatable if diagnosed early. Nephropathies or acute renal impairments can also occur due to infections, therapeutics, and other causes.
One of the most established methods to assess glomerular filtration rate and kidney function is the creatinine clearance. For calculation of the creatinine clearance, creatinine is simultaneously measured in serum and urine, collected over a defined period.
Cystatin C represents a superior alternative for the detection of reduced glomerular filtration rate (GFR) compared to creatinine. In contrast to creatinine, cystatin C is independent of factors such as sex, muscle mass, diet, most inflammatory processes, and age. Cystatin C is sensitive for the identification of mild to moderately impaired kidney function, therefore, suitable for the detection of early stages of kidney impairment, especially in the “creatinine blind-range” defined by a GFR of 40 to 80 mL/min/1.73m².
The kidneys have high retention for plasma proteins. Increased albumin concentrations in urine indicate bleedings in the urinary tract or infections of the renal pelvis. A small abnormal albumin excretion known as microalbuminuria serves as an early indicator of temporary overload or chronic injury of the glomeruli.
Additionally, it may be useful to monitor electrolytes such as sodium, potassium, and chloride. Urea and uric acid are two of the metabolic products of amino acid and purine catabolism that are filtered in the kidney. These parameters show abnormal levels in various conditions but can be affected by renal diseases as well.