Although the risk of developing progressive CKD in patients with isolated microscopichaematuria is low, renal or urinary pathology is often present. The Japanese study followed 165patients with persisting haematuria and 13 of 17 patients who underwent renal biopsy had IgAnephropathy.61A similar rate of IgA disease was detected in a UK biopsy study.
DEFINING GLOMERULAR FILTRATION RATEThe glomerular filtration rate is defined as the volume of plasma which is filtered by the glomeruliper unit time and is usually measured by estimating the rate of clearance of a substance fromthe plasma. Glomerular filtration rate varies with body size and conventionally is correctedto a body surface area （BSA） of 1.73 m2, the average BSA of a population of young men andwomen studied in the mid-1920s.
Historically, measurement of creatinine or urea in serum or plasma has been used to assesskidney function. Both are convenient but insensitive （GFR has to halve before a significant risein serum creatinine becomes apparent）。 In addition, serum concentrations of creatinine areaffected by various analytical interferences, and depend critically on muscle mass, for example,a serum creatinine concentration of 130 micromol/l might be normal in one individual butrequire further investigation in another.
Other factors which affect creatinine concentrations include age, sex, ethnicity, body habitusand diet.72-73Diet may have a rapid and transient effect on creatinine concentration74-76andthere is evidence that consumption of cooked meat, in particular, may affect CKD categorisationbased on estimated glomerular filtration rate （eGFR）。