Accuracy of Cyclosporin Measurements Made in Capillary Blood Samples Obtained by Skin Puncture

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Abstract
International consensus guidelines suggest that cyclosporin should be measured in whole blood. In some instances it may be advantageous to collect capillary blood, by a finger or ear prick method. However, drug concentrations in skin-puncture blood may not necessarily correlate with those measured in venous blood. This study compared cyclosporin concentrations in blood collected from the fingertip or earlobe with blood collected by standard venipuncture. Patient preference for each of the blood collection methods was also assessed. Specimens were obtained from organ transplant patients receiving cyclosporin, using each of the three methods: venipuncture, finger prick, and earlobe prick. The samples were assayed using a specific radioimmunoassay and the results were compared. In the 102 sets of samples collected, the mean difference (±standard deviation) in cyclosporin concentration between finger prick and venipuncture and ear prick and venipuncture was 2.6% (±9.5%) and 2.7% (±12.1%), respectively, while the comparable median (IQR) differences were 1.9% (−3.4% to +6.6%) and −1.1% (−2.8% to +7.2%), respectively. A high degree of correlation was observed between finger prick and venipuncture or ear prick and venipuncture or ear prick and finger prick (r2 > 0.86). Of the three methods of blood collection, finger prick was the patients' preferred method (P