In clinical practice, the glomerular filtration rate (GFR) is often estimated from
plasma creatinine. Several studies have shown cystatin C (cys C) to be a better parameter
for the diagnosis of impaired renal function. No data are available, however, on the
performance of cys C in follow-up of patients, compared with creatinine. Also, comparisons
of cys C with the Cockcroft and Gault (C&G) formula for estimation of GFR are few.
Plasma samples were obtained from 93 consecutive patients seen for GFR determination
and from 30 patients with diabetes mellitus type 2, of whom 23 were investigated a
second time after 2 years. GFR was determined with [125I]iothalamate. Plasma creatinine
was determined enzymatically and the creatinine clearance calculated according to
C&G. Cys C was measured with a particle-enhanced immunonephelometric method.
GFR correlated with 1/cys C (r = 0.873) as well as with C&G (r = 0.876). The area
under the curve (AUC) of the receiver operating curves (ROCs), a measure of diagnostic
accuracy, for cys C (0.931) and C&G (0.938) were equal (P = 0.815) and both better
than the creatinine AUC (0.848; P = 0.006). Bland and Altman analysis showed that
the simple formula GFR = -4.32 + 80.35 x 1/cys C, derived from our data, gave more
accurate (P < 0.0001) and more precise (P = 0.024) GFR estimates than obtained with
the C&G formula. The day-to-day variation (biological +analytical) for cys C was small
(3.1%, SD 2.51%) in diabetic patients. In the follow-up study in diabetic patients,
cys C was the parameter which had the best correlation (r = 0.66) with changes in
Cys C shows a high correlation with GFR. With a very simple formula, cys C gives a
good estimate of GFR, more accurate and precise than C&G. Because biological variation
is low, cys C gives also a good assessment of GFR changes during follow-up. Cys C
is the preferred endogenous parameter for GFR.