Neither urine output nor creatinine are good estimates of renal function. Creatinine clearance is more helpful and can be estimated by the equation:
Normal is around 100 ml per minute but this formula was derived for use in the steady state and, if the creatinine is rising, true clearance will be lower. In oedematous patients, the GFR can be overestimated as total body weight is greater than lean body mass. However, it is useful and can provide guidance for drug adjustments. Particularly in the elderly, with less muscle mass, "normal" creatinines can be deceptive. If an 80-year-old man of 60 kg has a creatinine of 100 jjmol litre-1
creatinine in |imol litre 1
Figure 8.2 The nephron (the functional unit of the kidney). A, afferent arteriole where ACE inhibitors act (angiotensin ll plays a role in autoregulation of renal blood flow by constricting the afferent blood vessels when there are low pressures); CD, collecting duct; DCT, distal convoluted tubule; E, efferent arteriole where NSAIDs act (prostaglandins play a role in regulation of renal blood flow and increase cortical flow; in a person on ACE inhibitors and NSAIDs who is admitted with vomiting and diarrhoea - or hypovolaemia of any cause - both these drugs should be temporarily stopped; the combination of hypovolaemia, lack of constriction in the afferent vessels, and lack of vasodilatation in the efferent vessels reduces GFR; in cirrhosis and congestive cardiac failure where prostaglandins are recruited to increase renal blood flow, NSAIDs are more potent); F, thick ascending limb of the loop of Henle where frusemide blocks active sodium-potassium-chloride cotransport (despite documented protective effects in experiments, frusemide has failed to prevent postoperative renal failure in high risk patients or ameliorate established acute renal failure in clinical practice); LH, thick and thin descending and ascending Loop of Henle; PCT, proximal convoluted tubule.
(12 mg dl although this is within the "normal" range, his actual creatinine clearance is 43 ml per minute.
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