RENAL FUNCTION TESTS
Renal function tests are used to detect the presence of renal diseases and assess their progress. They are, however, of little use in determining the causes of renal disease. The most widely used test is to measure the glomerular filtration rate (GFR), that is, the rate of filtrate formation by the kidneys. The value of the GFR depends on the net pressure across the glomerular membrane, the physical nature of the membrane and the surface area of the membrane that reflects the number of functioning glomeruli. All three factors can change as a result of disease and this will be reflected in the value of the GFR. In adults, the GFR is about 120 cm3 per minute although it is related to body size, being higher in men than women. The GFR is also affected by age and declines in the elderly.
The GFR is determined by measuring the concentration of a substance in the urine and plasma that is known to be completely filtered from the plasma at the glomerulus. This substance must not be reabsorbed nor secreted by renal tubules and must remain at a constant concentration in the plasma throughout the period of urine collection. It should also be possible to measure the concentration of this substance in the plasma and urine both conveniently and reliably. Inulin and creatinine have been used to assess GFR using the equation:
GFR (Uc xV) / Pc
where Uc is the concentration of substance in urine, Pc is the concentration of substance in plasma, V is rate of formation of urine in cm3 per min giving the GFR units of cm3 min–1.
Creatinine is derived from creatine phosphate in the muscle and the amount produced daily is relatively constant. An estimate of creatinine clearance can be made by a determination of the creatinine concentration in the plasma (Figure 1.17) and the creatinine content in a 24-h urine collection. Normal creatinine clearance in adults is between 115 and 125 cm3 min–1. Reliable measurements of creatinine clearance are often difficult because of the need to obtain a complete and accurately timed urine sample.
Measurements of the concentration of serum creatinine may be used to assess renal function and they are easier to determine than creatinine clearance values. The concentration of creatinine in serum increases with deteriorating renal function but this test lacks sensitivity. For example, the GFR must fall to less than 50% of the original value before there is a significant increase in serum creatinine. This means that a normal serum creatinine value does not necessarily exclude the presence of renal disease.