Minimum volume of discard for valid blood sampling from indwelling arterial cannulae

The volume of discard for valid blood sampling from indwelling femoral (leader cath) and radial (Venflon) arterial cannulae (with internal volume of cannula plus connecting tubing of approximately 2 ml) was investigated by analysing sequential 2-ml aliquots of 10-ml samples. The aliquots were analysed for pH, carbon dioxide tension, oxygen tension, standard bicarbonate, haemoglobin concentration, haemoglobin oxygen saturation and potassium concentration. Analysis of variance showed that, for these variables and these catheters, a valid blood sample was obtained after discarding 4 ml, but not after only 2 ml. The haemoglobin concentration, as measured by a Corning oximeter, provided good warning of inadequate discard.