Stability of 21 Antihypertensive Drugs in Serum Collected in Standard (Nongel) Serum Tubes Versus Tubes Containing a Gel Separator

BACKGROUND: Therapeutic drug monitoring of antihypertensive drugs is being increasingly used to optimize treatment and to assess nonadherence. Separator gels are often used in blood collection tubes to facilitate serum or plasma separation from other blood constituents before analyses. Drug adsorption into the separator gel presents a possible pre-analytical cause of falsely low concentrations or false negative results. METHODS: Drug-free blood from blood donors was spiked with therapeutic concentrations of 21 antihypertensive drugs, transferred to serum tubes with and without separator gel (Vacuette gel plastic tubes and plain serum plastic tubes, respectively), and centrifuged. Serum was collected immediately after centrifugation and after 24 and 72 hours of room temperature storage, samples were analyzed in triplicates using liquid chromatography–mass spectrometry. RESULTS: Serum samples collected immediately after centrifugation or 24 hours later, had the same drug concentrations in the gel and nongel tubes. After 72 hours of room temperature storage, verapamil and lercanidipine serum concentrations were 43% and 29%, respectively, lower in gel tubes than nongel tubes. Canrenone, diltiazem, and bendroflumethiazide showed between 10% and 20% concentration loss in gel tubes, compared with nongel tubes, with the 2 latter observed as unstable also in nongel tubes. CONCLUSIONS: Except for verapamil, lercanidipine, and canrenone, which showed substantial concentration loss in gel tubes, gel tubes may be used for therapeutic drug monitoring purposes for the most commonly used antihypertensive drugs. Transferring serum to gel-free containers immediately after centrifugation minimizes concentration loss; however, bendroflumethiazide and diltiazem are generally unstable at room temperature.