![]() ![]() Selection of the most appropriate test depends on the clinical question, the required accuracy and cost (Table 2). Both these methods are more expensive and less convenient to the patient. Measured creatinine clearance requires a 24-hour urinary collection while isotopic methods involve intravenous injection of a nuclear tracer, and two subsequent blood samples to estimate clearance. Serum creatinine or calculated creatinine clearance are the most convenient estimates of GFR, requiring only a single blood sample. Each method has its advantages and disadvantages in terms of accuracy, cost and convenience (Table 2). The GFR can be estimated from the serum concentration of filtration markers (such as creatinine or urea) or the renal clearance of these markers. When an ideal filtration marker is used (and there is no hepatic metabolism or non-renal clearance) and the input is constant (for example, by endogenous creatinine generation), then the plasma concentration is inversely proportional to the GFR. The plasma concentration of a substance in a steady state depends on the balance of the input (from either endogenous production or exogenous intake) and the clearance from the blood (by either excretion or metabolism). The clearance of ideal filtration markers can be shown mathematically to be an accurate estimate of GFR. Where U is the urinary concentration of an ideal filtration marker of x, V is the urine flow rate and P x is the average plasma concentration of x.Īn 'ideal filtration marker' is a substance that is freely excreted by glomerular filtration, without tubular reabsorption or secretion. The GFR cannot be directly measured in humans, but can be estimated from urinary clearance of a substance (x), given by the equation: These conditions should be considered when interpreting a patient's GFR. In addition to ageing there are a number of physiological and pathological conditions that can affect GFR, including pregnancy, hypertension, medications and renal disease. The corrected GFR is approximately 8% lower in women than in men, and declines with age at an annual rate of 1 mL/min/1.73m 2 from the age of 40. The normal corrected GFR is 80-120 mL/min/1.73m 2, impaired renal function is 30-80 mL/min/1.73m 2 and renal failure is less than 30 mL/min/1.73m 2. When the GFR is corrected for body surface area, a normal range can be derived to assess renal impairment. GFR is conventionally corrected for body surface area (which equates with renal mass), which in normal humans is approximately 1.73m 2 and represents an average value for normal young men and women. The GFR varies according to renal mass and correspondingly to body mass. Measurement of tubular function is impractical for daily clinical use, so we usually use the GFR to assess renal function. They control a number of kidney functions including acid-base balance, sodium excretion, urine concentration or dilution, water balance, potassium excretion and small molecule metabolism (such as insulin clearance). Renal tubules make up 95% of the renal mass, do the bulk of the metabolic work and modify the ultrafiltrate into urine. Unfortunately it is not an ideal index, being difficult to measure directly, and is sometimes insensitive for detecting renal disease.Īlthough glomeruli control the GFR, damage to the tubulointerstitium is also an important predictor of GFR and progression towards renal failure. The GFR can predict the signs and symptoms of uraemia, especially when it falls to below 10-15 mL/min. It is reduced before the onset of symptoms of renal failure and is related to the severity of the structural abnormalities in chronic renal disease. The GFR is a direct measure of renal function. Approximately 120 mL are formed per minute. Glomerular filtration rate (GFR) is the rate (volume per unit of time) at which ultrafiltrate is formed by the glomerulus. ![]() Renal damage or alterations in glomerular function affect the kidneys' ability to remove metabolic substances from the blood into the urine. It generates an ultrafiltrate that is free of blood and significant amounts of blood proteins. The glomerulus is a high-pressure filtration system, composed of a specialised capillary network. Renal function and glomerular filtration rate Table 1 Indications for renal function testing The dose of the drug may need to be adjusted if renal function is abnormal. Renal function should also be calculated if a potentially toxic drug is mainly cleared by renal excretion. Estimation of renal function is important in a number of clinical situations (Table 1), including assessing renal damage and monitoring the progression of renal disease. ![]()
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