The Effect of Sample Storage on Impedance and Optical Platelet Counts

Introduction: Processing blood samples for full blood count may be delayed for a multitude of reasons. There is little published information on the effect of sample storage on platelet count. This study compares the effect of storing normal samples at room temperature (RT) and at 4°C prior to processing using different analysers with different technologies. Sysmex SF 3000 and XE2100 analysers are capable of providing impedance (IMP) platelet counts and the latter can also perform fluorescence based optical (OPT) platelet counts. The Coulter LH750 and the ABX Pentra DX120 use IMP technology with enhanced data extraction techniques and algorithms to eliminate interference.

Method: Intravenous blood samples for routine blood counts were taken into 4ml K2 EDTA Greiner Vacuette containers from 20 out-patients and the platelet counts measured at 0, 6 and 24 hrs on Coulter LH750 (IMP), ABX Pentra DX120 (IMP), Sysmex SF3000 (IMP) and Sysmex XE2100 (both IMP and OPT) blood count analysers. Samples were stored at RT. Samples were also processed on the XE2100 following storage at 4°C.

Results: Mean changes in platelet counts (n = 20) expressed as percentages of the original value at 0 hrs and paired t-test values of significance are shown in the Table. The most significant changes occurred with the XE2100 where the IMP value increased by 7% over 24 hrs but the OPT value fell by 5%. No significant changes occurred with the Coulter or the ABX analysers at 0 and 24 hrs or with the SF3000 at 6 hrs.

EDTA exposure and the formation of membrane lesions during sample storage leads to changes in platelet structure, volume, function and shape due to decreased optical density and the beginning of degranulation. All these have a potential impact on the accurate assessment of platelet numbers. If these numbers are to be used accurately as thresholds for platelet transfusion or for monitoring progressive loss or increases of platelets associated with underlying pathology, it is vital that the pre-analytical limitations of current analysers are fully understood. The way in which discriminators are used on various analysers to make distinction between the characteristics of platelets and other blood cells in individual, stored samples will vary in their effectiveness.

Conclusions: Our data indicate that various combinations of pre-analytical sample storage and analyser technology should be considered as potential influences on the outcome of platelet counting in the clinical laboratory.