Rounding of low serum creatinine levels and consequent impact on accuracy of bedside estimates of renal function in cancer patients

To compare glomerular filtration rate measured by technetium-99m ([Tc(99m)]) DTPA clearance with estimated creatinine clearance (CrCl) (Cockcroft and Gault (C&G) method) in patients with serum creatinine (Scr) levels <0.06 mmol l(−1), and determine the effect of rounding serum creatinine to 0.06 mmol l(−1). Patients with serum creatinine values <0.06 mmol l(−1) at the time of [Tc(99m)] clearance determination were identified. Creatinine clearance was calculated by the C&G method using both actual and rounded Scr values. A total of 419 adults had GFR measured by technetium-99m diethyl triamine penta-acetic acid ([Tc(99m)] DTPA) clearance. Out of this group, 26 patients had a serum creatinine value <0.06 mmol l(−1). The C&G estimates of renal function using actual serum creatinine resulted in an overall overestimation of 12.9% when compared to [Tc(99m)] DTPA clearance. When the value of serum creatinine was rounded to 0.06 mmol l(−1), the formula underestimated renal function by −7.0%. Analysis of estimated creatinine clearance for different levels of renal function showed significant differences to [Tc(99m)] DTPA clearance. Rounding up of serum creatinine to 0.06 mmol l(−1) improved the predictive ability of the C&G method for the patients with [Tc(99m)] DTPA clearance ⩽100 ml min(−1), but worsened the effect in those >100 ml min(−1). This work indicates that when bedside estimates of renal function are calculated using the C&G formula actual Scr should be used first to estimate CrCl. If the resultant CrCl is ⩽100 ml min(−1), then the Scr should be rounded up to 0.06 mmol l(−1) and CrCl recalculated. Further assessment of this approach is warranted in a larger cohort of patients.