Does tube diameter influence the packed cell volume?

The relationship between observed PCV and PCV tube diameter was investigated and it was found that the PCV decreased as the PCV tube diameter decreased. An equation which predicted the PCV as a function of the PCV tube radius was derived from the hypothesis that red blood cells in contact with the glass wall were partially dehydrated during high-speed centrifugation and that the amount of dehydration was related to the type of glass. PCV determinations were made on five patient blood samples using eight tubes of varying diameters, four made of soda-lime glass and four made of borosilicate glass. The predictions of the equation correlated well with the experimental results. Twice as much dehydration was observed in the soda-lime tubes as in the borosilicate, thus confirming both parts of the hypothesis. Independent confirmation of the hypothesis was obtained by counting the number of spheroechinocytes in the bottom of the PCV tube. There were twice as many in the soda-lime as in the borosilicate glass tubes. We conclude that dehydration errors must be considered along with trapped plasma errors when attempting to perform a reference PCV determination.