Comparison of 50mmol/L and 150mmol/L concentration of sodium hydroxide in Jaffe's two point method for serum creatinine measurement with respect to linearity of assay

Objectives: Objective of the study is to perform linearity check for Creatinine measurement with Jaffe's two point method using CLSI EP-6 guideline for 50 mmol/L, 150mmol/L NaOH concentration in final reagent mixture. Material and Method: Leftover serum of samples received by the laboratory, devoid of patient identification, was used to make pool of serum. Serum creatinine reagents have NaOH of 50 mmol/L, 150 mmol/L were prepared. Serial dilution of pooled serum spiked with creatinine powder was prepared. Creatinines were measured in series of dilution. Linear regression analysis of each dilution series to find linearity limit was performed. Result and Discussion: This study evaluated Linearity difference plot using CLIA criteria (15% or 0.3 mg/dl whichever is higher) for creatinine at 95% confidence limit.7.5% of result or 0.15 mg/dl were used for plotting Linearity Difference plot. If any the two conditions are satisfied at a given point in the Linearity Difference Plot, the point is considered within linearity. 50 mmol/L NaOH in final reagent mixture gives limit of linearity 28.95 mg/dl and 150 mmol/L NaOH in final reagent mixture gives limit of linearity 31.16 mg/dl. Conclusion: 150 mmol/L final reagent concentration of NaOH gives higher linearity of 31.16 mg/dl, low slop (a=29, 23) indicating high sensitivity of the reagent, low y-intercept (b=-0.4578) indicating minimum effect of non-creatinine chromogens; i.e. high specificity than 50mmol/L final reagent concentration of NaOH.