Historically, serum creatinine was analyzed from a blood sample using a method called alkaline picrate. In addition to creatinine molecules, though, it also “counted” non-creatinine molecules that falsely elevated the resulting value by as much as 20%. This assay method was used for decades in the development of creatinine clearance estimates, such as the Cockcroft-Gault method.
Within the past 10-15 year, however, laboratories have largely moved to a new assay called IDMS (isotope dilution mass spectrometry). This method does not detect the non-creatinine molecules, which means that the IDMS value is often 10-20% lower than the more conventional assay. Because older equations, like Cockcroft-Gault, were created and validated using a non-IDMS assay, this poses a problem for estimating creatinine clearance (a surrogate for glomerular filtration rate) when using an IDMS-based lab assay.
Converting from IDMS to non-IDMS (Conventional)
It is possible to “convert” an IDMS-based serum creatinine value to a non-IDMS (conventional) value. We’ve created a simple calculator, available by clicking here, that calculates the conversion.
As you can see below, the percent different (red bars) is most significant with lower serum creatinine values. As serum creatinine exceeds about 1.4 mg/dL, the percent difference becomes less and less clinically relevant: