The kidneys are part of the urinary system and perform many different functions for the body. They include filtration and elimination of metabolic wastes, regulation of of water and electrolyte balance and conservation of nutrients such as glucose and amino acids. We assess renal function using chemistry results which indicate glomerular function via measurement of urea nitrogen and creatinine concentrations, primarily (see below) and urinalysis results, which mostly indicate tubular function (but protein in the urine can also reflect glomerular disease).
In this section we will cover:
- Renal physiology: How the kidney eliminates nitrogenous waste, resorbs filtered substances (glucose, electrolytes, amino acids) preventing their loss, maintains water homeostasis (creation of a hypertonic medulla), and modulates acid-base status.
- Types of renal disease: Acute kidney injury (transient, persistent), acute kidney and chronic kidney disease.
- Azotemia: The different types of azotemia (prerenal, renal and postrenal azotemia) and how to distinguish between them.
- Laboratory indicators of renal disease: These include markers of glomerular function, principally, glomerular filtration rate (GFR), such as urea nitrogen and creatinine. A urinalysis is an essential part of assessment of renal function as well and gives us a measure of tubular function or disease (see above). Since the kidney affects other biochemical test results, including proteins (particularly albumin) and electrolytes and minerals, these serum or plasma biochemical test results should be interpreted along with changes in more direct renal tests, indicating renal dysfunction.
- Markers of GFR: Urea nitrogen, creatinine, cystatin C, symmetrical dimethylarginine (SDMA). This is most accurately measured with clearance tests, such as inulin clearance.
- Urinalysis: In particular urine specific gravity (which tells us about tubular function), urine sediment examination (cells, crystals and casts), chemical constituents (tells us tubular and general body metabolic function) and gross features.