 Most clinical laboratories now align their creatinine measurements against a new standardized isotope dilution mass spectrometry (IDMS) method to measure serum creatinine.IDMS appears to give lower values than older methods when the serum creatinine values are relatively low, for example 0.7 mg/dL.The IDMS method would result in a comparative overestimation of the corresponding calculated GFR in some patients with normal renal function.