# IDMS to Conventional Serum Creatinine

## Converts IDMS values to the conventional serum creatinine assay

### IDMS Creatinine Assay

 IDMS Creatinine mg/dL µmol/L

RESULTS

#### Serum creatinine assay conversion:

 IDMS Conventional 1.3 mg/dL 1.45 mg/dL

#### Mathematical Calculation

$$\\ Conventional\;SCr\;(mg/dL) = (IDMS\;SCr)*1.065 + 0.067$$

In order to estimate a patient's glomerular filtration rate (GFR), a blood sample is collected and the amount of creatinine is measured. In conjunction with mathematical equations, usually Cockcroft-Gault (1976), a creatinine clearance is estimated, which correlates closely to GFR.

Historically, serum creatinine was measured using "conventional" (older) assay methods, such as the alkaline picrate method. Although extensively used, this assay detects "non-creatinine chromogens" (chemicals that are not creatinine) and results in a falsely elevated laboratory result.1

The NKDEP guidelines2 recommend that all laboratories convert their systems to use a newer, more accurate assay method called IDMS. IDMS does not detect non-creatinine chromogens, resulting in a more accurate, but lower serum creatinine value than the conventional assay.

#### Correction for Bias

Prior to widespread adoption of IDMS, modeling estimates of creatinine clearance were based on "conventional" assays. Estimates of glomerular filtration rate, such as the Cockcroft-Gault equation, were developed using the older assay that could be falsely elevated by 20% or higher compared to the more accurate IDMS method. In order to still use these classic equations to estimate creatinine clearance, an IDMS-based serum creatinine value should be "converted" to a conventional value using the following equation:3

$$\\ Conventional\;SCr\;(mg/dL) = (IDMS\;SCr)*1.065 + 0.067$$

#### Degree of Conventional-IDMS Agreement

As shown in the graph below, the percent difference between the conventional serum creatinine assay method and IDMS is greatest at low serum creatinine values (about 20% different) and progressively declines to a difference of about 10% for serum creatinine values above about 1.2 mg/dL. Given the inherent error in estimating creatinine clearance using Cockcroft-Gault, this percent difference is unlikely to have a clinical impact, but should be account for in electronic clinical decision support tools. All calculators on ClinCalc.com support the ability to specify a serum creatinine value as IDMS or non-IDMS.

#### The Future with IDMS

According to the NKDEP, almost all laboratories are expected to convert to the IDMS method by the end of 2010. This shift in laboratory standards has already been implemented in modern estimates of glomerular filtration rate. The MDRD equation, used to calculate "eGFR", was originally developed using a conventional serum creatinine assay and later was republished using IDMS-based values. The update to MDRD, called the CKD-EPI equation, exclusively relies on IDMS-based values only.

1. Spruill WJ, Wade WE, Cobb HH 3rd. Estimating glomerular filtration rate with a modification of diet in renal disease equation: implications for pharmacy. Am J Health Syst Pharm. 2007;64(6):652-60. PMID 17353576.
2. National Kidney Disease Education Program. Chronic Kidney Disease and Drug Dosing: Information for Providers. Available at http://www.nkdep.nih.gov/.../CKD_DrugDosing_508.pdf. Updated January 2010. Accessed December 18, 2011.
3. Ortho-Clinical Diagnostics. Updated Information for IDMS Traceable VITROSĀ® Chemistry Products CREA Slides. June 12, 2008. Written communication available for download.