Serum creatinine

Serum creatinine concentration, often used as a measure of renal function, has poor sensitivity because levels may remain within the normal range until renal function is compromised by more than 50%, particularly misleading in pregnancy if the lower average serum levels are not appreciated (Table 23.2). In addition, serum creatinine levels are influenced by diurnal variation and dietary factors. An isolated serum creatinine result above 75 mmoll-1 in pregnancy should prompt further renal investigation but such patients may have considerable renal compromise by this point.

Although para-aminohippurate (PAH) and inulin clearances remain the gold standard to assess renal function, impracticality excludes their routine performance in pregnancy. An alternative, the commonly used 24 h creatinine clearance, relies upon creatinine being freely filtered at the glomerulus and minimal tubular secretion. Consequently the increased renal perfusion in pregnancy causes the 24 h creatinine clearance to increase from nonpregnant mean values of 92mlmin-1 to maximal gestational levels of 125mlmin-1 in the mid trimester. Creatinine clearance is an approximation of function because it is assumed that the tubular secretion of creatinine is offset by the overesti-mation of plasma creatinine levels as a result of technical limitations of creatinine assays

Table 23.2. Mean serum levels of urea and creatinine in pregnancy

Nonpregnant First Second Third trimester trimester trimester

Table 23.2. Mean serum levels of urea and creatinine in pregnancy

Nonpregnant First Second Third trimester trimester trimester

Serum

73

65

51

47

creatinine

(mmol T1)

Serum urea

4.3

3.5

3.3

3.1

(mmol T1)

(chromagen is measured along with true creati-nine). Under normal circumstances this assumption is valid but if GFR falls considerably creatinine clearance may overestimate GFR by as much as 25-50% (Smith, 1951).

Several groups have compared gestational GFR estimation using both inulin and creatinine clearances and the results are generally comparable with the possible exception of late pregnancy where 24 h creatinine clearance falls (Davison and Hytten, 1974; de Alvarez, 1958). This is not just due to posture-dependent pressure of the enlarged uterus on the renal vasculature but also there is some renal ''de-adaptation'' in the third trimester.

Clinicians caring for nonpregnant patients may be used to applying formulae to correct for patient age, height, sex and weight, which are not appropriate in pregnancy because body weight and size do not reflect an increase in functional renal mass (Conrad et al., 1999; Hytten and Leitch, 1971).

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