Glomerular filtration rate refers to the rate in ml/min at which a substance is cleared from the circulation by the glomeruli. The ability of the glomeruli to filter a substance from the blood is assessed by clearance studies. If a substance is not bound to protein in plasma, is completely filtered by the glomeruli, and is neither secreted nor reabsorbed by the tubules, then its clearance rate is equal to the glomerular filtration rate (GFR). Clearance of a substance refers to the volume of plasma, which is completely cleared of that substance per minute; it is calculated from the following formula:
Clearance = UV
where, U = concentration of a substance in urine in mg/dl; V = volume of urine excreted in ml/min; and P = concentration of the substance in plasma in mg/dl. Since U and P are in the same units, they cancel each other and the clearance value is expressed in the same unit as V i.e. ml/min. All clearance values are adjusted to a standard body surface area i.e. 1.73 m2.
The agents used for measurement of GFR are:
- Exogenous: Inulin, Radiolabelled ethylenediamine tetraacetic acid (51Cr- EDTA), 125I-iothalamate
- Endogenous: Creatinine, Urea, Cystatin C
The agent used for measurement of GFR should have following properties: (1) It should be physiologically inert and preferably endogenous, (2) It should be freely filtered by glomeruli and should be neither reabsorbed nor secreted by renal tubules, (3) It should not bind to plasma proteins and should not be metabolized by kidneys, and (4) It should be excreted only by the kidneys. However, there is no such ideal endogenous agent.
Clearance tests are cumbersome to perform, expensive, and not readily available. One major problem with clearance studies is incomplete urine collection.
Abnormal clearance occurs in: (i) pre-renal factors: reduced blood flow due to shock, dehydration, and congestive cardiac failure; (ii) renal diseases; and (iii) obstruction to urinary outflow.
Inulin, an inert plant polysaccharide (a fructose polymer), is filtered by the glomeruli and is neither reabsorbed nor secreted by the tubules; therefore it is an ideal agent for measuring GFR. A bolus dose of inulin (25 ml of 10% solution IV) is administered followed by constant intravenous infusion (500 ml of 1.5% solution at the rate of 4 ml/min). Timed urine samples are collected and blood samples are obtained at the midpoint of timed urine collection. This test is considered as the ‘gold standard’ (or reference method) for estimation of GFR. However, this test is rarely used because it is time consuming, expensive, constant intravenous infusion of inulin is needed to maintain steady plasma level, and difficulties in laboratory analysis. Average inulin clearance for males is 125 ml/min/1.73 m2 and for females is 110 ml/min/1.73 m2. In children less than 2 years and in older adults, clearance is low. This test is largely limited to clinical research.
Clearance of Radiolabeled Agents
Urinary clearance of radiolabeled iothalamate (125Iiothalamate) correlates closely with inulin clearance. However, this method is expensive with risk of exposure to radioactive substances. Other radiolabelled substances used are 51Cr-EDTA and 99Tc-DTPA.
Cystatin C Clearance
This is a cysteine protease inhibitor of MW 13,000, which is produced at a constant rate by all the nucleated cells. It is not bound to protein, is freely filtered by glomeruli and is not returned to circulation after filtration. It is a more sensitive and specific marker of impaired renal function than plasma creatinine. Its level is not affected by sex, diet, or muscle mass. It is thought that cystatin C is a superior marker for estimation of GFR than creatinine clearance. It is measured by immunoassay.
This is the most commonly used test for measuring GFR.
Creatinine is being produced constantly from creatine in muscle. It is completely filtered by glomeruli and is not reabsorbed by tubules; however, a small amount is secreted by tubules.
A 24-hour urine sample is preferred to overcome the problem of diurnal variation of creatinine excretion and to reduce the inaccuracy in urine collection.
After getting up in the morning, the first voided urine is discarded. Subsequently all the urine passed is collected in the container provided. After getting up in the next morning, the first voided urine is also collected and the container is sent to the laboratory. A blood sample for estimation of plasma creatinine is obtained at midpoint of urine collection. Creatinine clearance is calculated from (1) concentration of creatinine in urine in mg/ml (U), (2) volume of urine excreted in ml/min (V) (this is calculated by the formula: volume of urine collected/collection time in minutes e.g. volume of urine collected in 24 hours ÷ 1440), and (3) concentration of creatinine in plasma in mg/dl (P). Creatinine clearance in ml/min per 1.73 m2 is then derived from the formula UV/P.
Because of secretion of creatinine by renal tubules, the above formula overestimates GFR by about 10%. In advanced renal failure, secretion of creatinine by tubules is increased and thus overestimation of GFR is even more.
Jaffe’s reaction (see serum creatinine) used for estimation of creatinine measures creatinine as well as some other substances (non-creatinine chromogens) in blood and thus gives slightly higher result. Thus effect of tubular secretion of creatinine is somewhat balanced by slight overestimation of serum creatinine by Jaffe’s reaction.
To provide values closer to the actual GFR, cimetidine (which blocks secretion by renal tubules) can be administered before commencing urine collection (cimetidine-enhanced creatinine clearance).
Creatinine clearance is not an ideal test for estimation of GFR because of following reasons:
- A small amount of creatinine is secreted by renal tubules that increase even further in advanced renal failure.
- Collection of urine is often incomplete.
- Creatinine level is affected by intake of meat and muscle mass.
- Creatinine level is affected by certain drugs like cimetidine, probenecid, and trimethoprim (which block tubular secretion of creatinine).
Urea is filtered by the glomeruli, but about 40% of the filtered amount is reabsorbed by the tubules. The reabsorption depends on the rate of urine flow. Thus it underestimates GFR, depends on the urine flow rate, and is not a sensitive indicator of GFR.
BUN and serum creatinine, by themselves, are not sensitive indicators of early renal impairment since values may be normal e.g. if baseline values of serum creatinine is 0.5 mg/dl, then 50% reduction in kidney function would increase it to 1.0 mg/dl. Thus clearance tests are more helpful in early cases. If biochemical tests are normal and renal function impairment is suspected, then creatinine clearance test should be carried out. If biochemical tests are abnormal, then clearance tests need not be done.