At PTB, a novel measuring set-up has been developed to calibrate syringes made of glass and plastic. Appropriate plastic syringes now provide more accurate reference values for interlaboratory tests with regard to an important analytical method of laboratory medicine – the determination of the complete blood count (CBC) or full blood count (FBC).
For external quality assurance, haematology laboratories have to take part in interlaboratory tests that are organised by medical professional associations. Thereby, PTB‘s task is to provide reference values for the complete blood count (CBC) or full blood count (FBC), i.e. to specify the concentration of the different blood cells present in a sample. Such values provide the physicians with valuable information about numerous diseases where, for example, the number of leucocytes is increased. Thereby, the different cell types – such as, e.g., red and white blood cells – are differentiated according to their size and structure and counted by means of a flow cytometer.
To measure the concentration of the different cell types in the sample, it is essential to precisely determine the volume of the blood sample. The fact that cells remain stuck on the walls of glass syringes and are, thus, lost for the measurement can, in the case of red blood cells, lead to a loss of up to 5 %. With plastic syringes made of polypropylene, cell loss is considerably lower; they have, however, not been used so far due to their unknown accuracy for volume determination in the microlitre range. The experiments performed by PTB have shown that, in the case of a volume of 10 microlitres, the syringes of selected manufacturers comply with the uncertainty of < 0.3 % specified in DIN standards on cell counting. They are thus suitable for the determination of cell concentrations as long as the plastic plunger is replaced with a plunger made of aluminium.
Essential features of the new measuring set-up are an integrated, newly developed evaporation trap and the application of an appropriate measurement cycle with regard to the correction for the (remaining) evaporation. It is, thus, possible to achieve measurement uncertainties of approx. 0.1 % at a volume of 10 µL. In future, the sensitivity of the arrangement will be increased to be able to determine also volumes in the range below 1 µL with low measurement uncertainties.
Reitz, S.; Kummrow, A.; Kammel, M.; Neukammer, J.:
Determination of micro-litre volumes with high accuracy for fl ow cytometric blood cell counting. Measurement Science and Technology, Vol. 21, 074006 (9pp), 2010