The non-SI ratio (BCR) is BUN (mg/dL) / plasma creatinine (mg/dL). These two different ways of reporting urea results give rise to two quite different values for the ratio. In all other parts of the world, urea results are expressed as the whole urea molecule (MW 60) and reported as urea in SI units (mmol/L). In the US and a few other countries, urea concentration is expressed as the nitrogen content of urea (MW 28) and reported as blood urea nitrogen (BUN) in non-SI units (mg/dL). The distinction between BUN:creatinine ratio (BCR) and urea:creatinine ratio (UCR) Interpretation of increased plasma urea is thus often aided by simultaneous measurement of creatinine and calculation of the urea: creatinine ratio in order to establish a renal or non-renal cause. renal disease) is also associated with increased plasma urea concentration, there are other non-renal conditions that can give rise to increased plasma urea. Increased plasma creatinine is almost invariably a consequence of reduced GFR and therefore has a renal cause. By comparison with urea, however, creatinine more closely fulfills the above criteria and for this reason is the preferred test for assessment of kidney function. Typically, GFR must be reduced by ~50 % before plasma urea or creatinine concentration rise above the upper limits of their respective reference range. it is only affected by change in GFR)īoth plasma urea and plasma creatinine concentration are imperfect indices of GFR neither analyte entirely fulfills the above criteria (see Table I below) and both lack sensitivity to detect minimal change in GFR. Blood concentration of the substance must be unaffected by diet and/or change in the rate of endogenous.all that is filtered at the glomerulus appears in urine, and all that is in urine is due to glomerular filtration) It must be neither reabsorbed from the filtrate to blood nor secreted from blood to the filtrate by renal tubulecells (i.e.It must be freely filtered from blood at the glomerulus.It must be excreted only by the kidneys. kidney function) declines, urinary excretion of urea and creatinine also declines and blood concentration of both increases.įor the blood concentration of an endogenously produced substance to most accurately reflect GFR in health and disease, that substance must have the following properties: Irrespective of its cause, kidney disease is associated with decrease in GFR, and the severity of kidney disease correlates closely but inversely with GFR.Ī normal GFR (~125 mL/min) is presumptive evidence of healthy, functioning kidneys. The rationale for the use of creatinine or urea measurement to assess renal function is that plasma/serum levels of both reflect glomerular filtration rate (GFR), the parameter that defines kidney function for the clinician. Creatinine and urea blood levels reflect glomerular filtration rate (GFR)
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