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CME Renal medicine Clinical Medicine 2009, Vol 9, No 3: 269–72 Clinical management tified on imaging performed for other reasons. to adjust for dilution of the urine. Spot urine samples for total protein-creati- nine ratio (TPCR) or albumin-creati- of chronic kidney nine ratio (ACR) perform at least as Estimated glomerular filtration rate well as 24-hour urine samples in most disease Although serum creatinine (SCr) is a good circumstances and are far more conve- marker of change in GFR in an individual nient.7 The first morning void is pre- Shona Methven, Specialty Registrar and patient, it is a poor measure of absolute ferred but random samples are Clinical Teaching Fellow; Mark S MacGregor, GFR.6 One major flaw is that in addition satisfactory. Consultant Nephrologist to correlating inversely with GFR, SCr also ACR has a proven role in diabetic The John Stevenson Lynch Renal Unit, NHS correlates with muscle mass. eGFR is a cal- kidney disease and should be used to Ayrshire & Arran, Crosshouse Hospital, culated value derived from SCr, age, screen for and monitor this disease. Kilmarnock gender and race6 in which these three Whether TPCR or ACR should be used parameters essentially provide a correction in non-diabetic kidney disease is con- Chronic kidney disease (CKD) is a major for muscle mass (commonly used equa- troversial, with guidelines making dif- public health problem which commonly tions are given in Table 2). If muscle mass fering recommendations.4,5 Most presents to primary and secondary care. differs substantially from the average for research on outcomes and interven- Reporting of estimated glomerular filtra- age, race and gender, the eGFR will be less tions7,8 is based on total proteinuria tion rate (eGFR) has markedly increased accurate. Common examples include rather than albuminuria so the theoret- identification of these patients. Prevalence amputees and the malnourished. In such ical advantages of the more costly ACR may be genuinely rising, perhaps as a con- patients, a 24-hour urinary creatinine remain unproven. sequence of diabetes mellitus and obesity.1 clearance may still be of value. eGFR has CKD is primarily a marker of cardiovas- been validated in whites and black Haematuria cular risk: stage 3 CKD carries a 40–100% Americans but its reliability is unproven in increased risk of cardiovascular events.2 other races. eGFR should not be used in Non-visible haematuria may be detected For the minority at risk of progressive children, in pregnancy, the very elderly on dipstick urinalysis. If persistent, and decline in kidney function, prompt identi- and those at both extremes of weight. It is after urological causes have been excluded, fication is necessary to allow early inter- unreliable in liver failure. it should be considered a marker of kidney vention and prevent complications. eGFR is increasingly inaccurate above damage.9 It does not require confirmation 60 ml/min/1.73 m2 and many laboratories with microscopy but warrants ongoing Classification do not report eGFR above that level. monitoring. Higher changes in kidney function should The CKD classification system originated be monitored using SCr, with a change of Investigations in the USA and was quickly adopted inter- more than 10–15% likely to be significant. nationally.3 The modified version used by SCr is increased by the ingestion of cooked Having identified CKD on the basis of UK guidelines is shown in Table 1.4,5 It is meat, so confirmatory samples are best an eGFR, a urine abnormality or based on the GFR and the presence or taken after abstaining from meat for imaging, an attempt should be made to absence of kidney damage. The latter is 12 hours. Like SCr, eGFR reflects GFR only establish the underlying diagnosis and loosely defined as the persistent presence in steady state, so both are unreliable when prognosis. Accurate diagnosis may lead of proteinuria (including microalbumin- kidney function is changing rapidly (eg in to specific therapies, in addition to gen- uria), haematuria or structural disease of acute kidney injury (AKI)). Having newly eral management (discussed below). the kidney (whether defined with imaging identified a reduced eGFR, it is important Conversely, extensive investigation of or histology). The reduced GFR and/or to exclude AKI with a further sample asymptomatic patients with stable CKD damage must be present for more than within 1–2 weeks. To establish the diag- stage 3, no proteinuria and no haema- 90 days to establish chronicity. (Note that nosis of CKD requires a further sample at turia is unlikely to be rewarding. Most stages 1 and 2 require the presence of least 90 days later. of these patients will have simple kidney damage whereas a reduced GFR glomerulosclerosis. More attention is alone is sufficient for stages 3–5.) warranted, however, if vascular disease Proteinuria or risk factors are absent. Identification and staging Di
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