Precise pH measuring of platelet concentrates containing additive solution--the impact of the temperature

Blood gas analysers measuring pH at 37°C (pH37) are widely used for pH determination of platelet (PLT) concentrates (PCs). For reporting pH at 22°C (pH22), converting of pH37 using the correct conversion factor is mandatory. For PCs stored in PLT additive solution (PAS), such conversion factors are not yet widely available. We studied pH in samples of PCs with different PAS/plasma ratios during warming from 22 to 37°C.

We measured pH in 39 samples containing modified PAS-III (PAS-IIIM) with a plasma carryover of 20%, 30% or 40% or no PAS-IIIM. Differences between pH22 and pH37 (dpH) were compared within and between study groups. Correlation between pH22 and dpH was tested. Additional measurements in 33 samples with three different PLT counts were performed to study the influence of PLT count on dpH.

pH22 and pH37 within each group and dpH or dpH/dT between study groups differed significantly. The dpH was 0·135 ± 0·040, 0·021 ± 0·009, 0·033 ± 0·011 and 0·048 ± 0·017 for samples containing 100%, 20%, 30% or 40% plasma, respectively. Correlation between dpH and pH22 was strong in 100% (r = 0·696, P < 0·001), weaker in 30% and 40% (r = 0·367, P = 0·022 and r = 0·345, P = 0·032, respectively) and not existing in 20% plasma (r = 0·153, P = 0·354). PLT count did not influence the dpH significantly.

The dpH is dependent on different PAS-IIIM/plasma ratios and pH range. For precise reporting of pH22, the respective dpH must be used if converting is necessary. Preferably, the pH should be reported at 37°C or measured directly at 22°C.