Clinical Validation of Simultaneous Analysis of Tacrolimus, Cyclosporine A, and Creatinine in Dried Blood Spots in Kidney Transplant Patients

BACKROUND: Monitoring of creatinine and immunosuppressive drug concentrations such as tacrolimus (TaC) and cyclosporin A (CsA) is important in the outpatient follow-up of kidney transplant recipients. Monitoring by Dried Blood Spot (DBS) provides patients the opportunity to sample a drop of blood from a fingerprick at home, which can be send to the laboratory by mail.

METHODS: We performed a clinical validation in which we compared measurements from whole blood samples obtained by venapuncture with measurements from DBS samples simultaneously obtained by fingerprick. After exclusion of 10 DBS for poor quality, and 2 for other reasons, 199, 104 and 58 samples from a total of 172 patients were available for validation of creatinine, TaC and CsA respectively. Validation was performed by means of Passing & Bablok regression and bias was assessed by Bland-Altman analysis.

RESULTS: For creatinine we found y = 0.73x - 1.55 (95% Confidence Interval [95%CI] slope 0.71,0.76), giving the conversion formula: [creatinine plasma concentration in μmol/L] = [creatinine concentration in DBS in μmol/L] / 0.73, with a nonclinically relevant bias of -2.1 μmol/L (95%CI -3.7,-0.5 μmol/L). For TaC we found y = 1.00x - 0.23 (95%CI slope 0.91,1.08), with a nonclinically relevant bias of -0.28 μg/L (95%CI -0.45,-0.12 μg/L). For CsA we found y = 0.99x - 1.86 (95%CI slope 0.91,1.08) and no significant bias. Therefore, for neither TaC nor CsA a conversion formula is required.

CONCLUSIONS: DBS sampling for the simultaneous analysis of immunosuppressants and creatinine can replace conventional venous sampling in daily routine.