# Mathematical Equations to Calculate True Mycophenolic Acid Concentration in Human Plasma by Using Two Immunoassays With Different Cross-Reactivities With Acyl Glucuronide Metabolite: Comparison of Calculated Values With Values Obtained by Using an HPLC-UV Method

### Abstract

Background

Both immunoassays and chromatographic methods are available for therapeutic drug monitoring of mycophenolic acid (MPA). Although chromatographic methods are more precise, immunoassays are widely used in clinical laboratories due to ease of adopting such assays on automated analyzers. We studied the possibility of using mathematical equations to calculate true MPA concentration by accounting for acyl glucuronide cross-reactivities with immunoassays by using two immunoassays with widely different cross-reactivities with the metabolite.

Methods

We determined MPA concentrations in 20 specimens obtained from transplant recipients using cloned enzyme donor immunoassay (CEDIA) assay and a new particle enhanced turbidimetric inhibition immunoassay (PETINIA) assay. Then we developed mathematical equations to calculate true MPA concentration using values obtained by both immunoassays and reported cross-reactivity of acyl glucuronide with respective immunoassays. Calculated concentrations were compared with values obtained by using a high-performance liquid chromatography combined with ultraviolet detection (HPLC-UV) method.

Results

We obtained good correlation between calculated MPA concentrations and corresponding MPA level obtained by using HPLC-UV method. Using x-axis as the MPA concentrations determined by the HPLC-UV method and y-axis as the calculated MPA level, we observed the following regression equation: y = 1.083x - 0.0995 (r = 0.99, n = 20).

Conclusions

Mathematical equations can be used to calculate true MPA concentrations using two immunoassays with different cross-reactivities with acyl glucuronide metabolite.