The kidneys, together with the liver, are the major organs responsible for the removal of waste material from the body. The kidneys also have other specific functions, including the control of electrolyte and water homeostasis, as well as the synthesis of erythropoietin. Each of the two kidneys contains approximately 1 million nephrons that receive the blood flowing to the kidneys. Blood flowing to the kidneys is first presented to the glomerulus of each nephron, which filters the plasma fluid to produce the ultrafiltrate or primary urine by removing all the contents of the plasma except proteins. Each nephron produces approximately 100 mm 3 of primary urine per day giving a total production of primary urine by the two kidneys of approximately 100–140 ml per minute or 200 litres per day in a healthy adult person. This is referred to as the glomerular filtration rate (GFR). The primary urine then encounters the tubule of the nephron that is the site of the reabsorption of water, the active and passive reabsorption of lipophilic compounds and cellular nutrients such as sugars and amino acids, and the active secretion of others. These two processes in combination result in the production of approximately 2 litres of urine per day, which is collected in the urinary bladder.
The glomerular filtration rate is the accepted best indicator of kidney function. Any pathology of the kidneys is reflected in a decreased GFR and this in turn has serious physiological consequences, including anaemia and severe cardiovascular dis ease. Kidney disease is progressive and proceeds through subacute or intrinsic renal disease, such as glomerular nephritis, into chronic kidney disease (CKD). Complete kidney failure leads to the need for kidney dialysis and organ transplantation. There is evidence that the incidence of CKD is increasing in developed countries and is associated with increasing risk of diabetes and an increasingly elderly population. There is thus a great clinical demand for accurate measurements of GFR in order to detect the onset of kidney disease, to assess its severity and to monitor its subsequent progression.
Measurement of Glomerular Filtration Rate
The measurement of GFR is based on the concept of renal clearance, which is defi ned as the volume of serum cleared of a given substance by glomerular filtration in unit time. It therefore has units of ml min−1 . In principle, any endogenous or exogenous substance that is subject to glomerular filtration and is not reabsorbed could form the basis of the measurement. The polysaccharide inulin meets these criteria and is subject to few variables or interferences. However, because it is not naturally occurring in the body, it is inconvenient for routine clinical use; it is commonly used as a standard for alternative methods. In practice, serum creatinine is the most commonly used marker. It is the end product of creatine metabolism in skeletal muscle and meets the excretion criteria, so that its serum concentration is inversely related to GFR. However, it is subject to a number of non-renal variables, including:
• Muscle mass: Serum values are influenced by extremes of muscle mass as in athletes and in individuals with muscle-wasting disease or malnourished patients.
• Gender: Serum creatinine is higher in males than females for a given GFR.
• Age: Children under 18 years have a reduced serum creatinine and the elderly have an increased value.
• Ethnicity: African–Caribbeans have a higher serum creatinine for a given GFR than Caucasians.
• Drugs: Some commonly used drugs such as cimetidine, trimethoprim and cephalosporins interfere with creatinine excretion and hence give elevated GFR values.
• Diet: Recent intake of red meat and oily fish can raise serum creatinine levels.
Routine laboratory estimations of GFR (referred to as eGFR) are based on the measurement of serum creatinine concentration and then the calculation of eGFR using an equation that makes corrections for four of the above variables. Serum creatinine is routinely measured by one of two ways:
• Spectrophotometric method based on the Jaffe reaction: This involves the use of alkaline picric acid reagent, which produces a red-coloured product that is measured at 510 nm. A limitation is that the reagent also reacts with other substances such as ketones, ascorbic acid and cephalosporins and, as a result, gives high values.
• Coupled enzyme assay: One method uses creatininase, creatinase and sarcosine oxidase to produce hydrogen peroxide. The hydrogen peroxide, in the presence of 4-aminophenazone, tri-iodo-3-hydroxybenzoic acid ( HTIB) and peroxidase, yields a quinone imine chromophore, whose colour intensity is directly proportional to the creatinine concentration:
Two other methods are commonly used for research:
• HPLC or GC-MS: HPLC uses a C18 column and water/acetonitrile (95:5 v/v) eluent containing 1-octanesulfonic acid as a cation-pairing agent. GC-MS is based on the formation of the tert -butyldimethylsilyl derivative of creatinine.
• Isotopic dilution coupled with mass spectrometry ( ID-MS). This involves the addition of 13C- or 15N-labelled creatinine to the serum sample, isolation of creatinine by ion-exchange chromatography and quantification by mass spectrometry using selected ion monitoring ( SIM). The lower limit of detection is about 0.5 ng.
The lack of an internationally or even nationally agreed standard assay for creatinine leads to significant inter-laboratory differences in both bias and imprecision so that national external quality assurance schemes, such as UK NEQAS and WEQAS have important roles in alerting laboratories to assays that stray out side national control values. In the UK, biological reference materials with ascertained concentrations are often provided by the National Institute for Biological Standards and Control (NIBSC). UK NEQAS provides clinical laboratories that participate in the eGFR scheme with an assay-specific adjustment factor to correct for methodological variations in estimations of serum creatinine. The factor is obtained using calibration against a GC-MS creatinine assay. It is updated at 6-monthly intervals. A number of equations have been derived to calculate eGFR from serum creatinine values, but the one currently used throughout the UK is the four-variable isotope dilution mass spectrometry ( ID-MS) traceable Modification of Diet in Renal Disease (MDRD) Study equation:
Serum creatinine concentrations are expressed in μM to the nearest whole number and are adjusted for variations in body size by normalising using a factor for body surface area (1.73 m2). The units of eGFR are therefore ml min −1 (1.73 m2) −1 and values are reported in whole numbers. The equation has been validated in a large-scale study against the most accurate method to measure GFR based on the use of 125 I-iothalamate. This equation should not be used in individuals with extremes of muscle mass, e.g. amputees, or those with unstable renal function or pregnant. Alternative equations exist for use with children.
Reference values for eGFR are 130 ml min −1 (1.73 m2) −1 for males in the age range 20–30 and 125 ml min −1 (1.73 m2) −1 for females of the same age range, although in practice, values greater than 90 are simply reported as > 90 ml min −1 (1.73 m2) −1 . Values decline with increasing age.
Clinical Assessment of Renal Disease
Acute kidney injury is the failure of renal function over a period of hours or days and is defined by increasing serum creatinine and urea. It is a life-threatening disorder caused by the retention of nitrogenous waste products and salts such as sodium and potassium. The rise in potassium may be visible by changes in the electrocardiogram and pose a risk of cardiac arrest. Acute kidney injury may be classified into pre-renal, renal and post-renal. Prompt identification of pre- or post-renal factors and appropriate treatment action may allow correction before damage to the kidneys occurs. Pre-renal failure occurs due to a lack of renal perfusion. This situation can occur as a result of volume loss in haemorrhage, gastrointestinal fluid loss and burns, or because of a decrease in cardiac output caused by cardiogenic shock, massive pulmonary embolus or cardiac tamponade (application of pressure) or other causes of hypertension such as sepsis. Post-renal causes include bilateral uretic obstruction because of calculi or tumours or by decreased bladder outflow/urethral obstruction, e.g. urethral stricture or prostate enlargement through hypertrophy of carcinoma. Correction of the underlying problem can avoid any kidney damage. Renal causes of acute renal failure include glomerular nephritis, vascular disease, severe hypertension, hypercalcaemia, invasive disorders such as sarcoidosis or lymphoma and nephrotoxins including animal and plant toxins, heavy metals, aminoglycosides, antibiotics and non-steroidal anti-inflammatory drugs.
Chronic kidney disease (CKD) is a progressive condition affecting both glomerular and tubular function and is characterised by a declining eGFR (Table 1). All CKD patients are subject to regular clinical and laboratory assessment and, once Stage 3 has been reached, to additional clinical management. This monitoring is aimed at attempting to reverse or arrest the disease by drug therapy, e.g. to treat hypertension, saving the patient the inconvenience and the paying authority the cost of dialysis or transplantation.
Table1. Stages of chronic kidney disease (CKD)
