Fingerprick blood samples can be used to accurately measure tacrolimus levels by tandem mass spectrometry

<jats:p><jats:bold>Abstract: </jats:bold> Regular monitoring of tacrolimus levels is an essential component of post‐transplantation follow up in children receiving this drug. We have developed a high performance liquid chromatography tandem mass spectrometry (HPLC T‐MS) methodology which measures whole blood tacrolimus levels using a 10 <jats:italic>μ</jats:italic>L sample, thus allowing the use of fingerprick blood sampling. The aim of this study was to determine the degree of relationship between fingerprick and venous tacrolimus levels measured by HPLC T‐MS and venous levels measured using the Abbott IMx tacrolimus II assay (IMx). This has not previously been investigated. Blood samples were collected from children on five separate occasions. In addition to the routine venous IMx sample, a further venous and fingerprick sample were collected for tacrolimus level measurement by HPLC T‐MS. These were mailed to the central laboratory. One hundred and seventy‐two sets of triplicate samples were collected from 36 children (33 kidney, two kidney–pancreas and one kidney–heart). Linear regression analysis showed highly significant relationships between HPLC T‐MS venous and IMx venous levels (r<jats:sup>2</jats:sup> = 0.83), HPLC T‐MS fingerprick and HPLC T‐MS venous levels (r<jats:sup>2</jats:sup> = 0.85) and HPLC T‐MS fingerprick and IMx venous levels (r<jats:sup>2</jats:sup> = 0.71) (all p &lt; 0.0001). Bland Altman analysis showed a small, although statistically significant difference between measured values, fingerprick HPLC T‐MS levels being lower than venous IMx levels, the mean difference being 0.58 ng/mL (95% CI 0.26–0.91, t = 3.54, d.f. = 168, p = 0.0005 one sample t‐test). Precise conversion between the two techniques could be achieved using the regression formula; IMx venous level = 0.751 + 0.978. HPLC T‐MS fingerprick level. There is a strong significant relationship between fingerprick blood tacrolimus levels measured by HPLC T‐MS and venous blood levels measured by IMx, allowing the two to be used interchangeably for routine clinical purposes.</jats:p>