Incomplete recovery of sodium determined in direct potentiometry in blood samples added with heparin salts.

The recent introduction on the market of apparatus for combined blood gas-electrolyte and ionized calcium analyses has raised the problem of whether the same sample can be submitted to all the analyses without generating problems for some analytes. The problem is basically connected to the common use of heparin as an anti-coagulant. To elucidate the possible effect of heparin on measurements we added pooled sera to tubes containing dried heparin/sodium chloride in increasing quantity. Our data demonstrate that, for one direct potentiometric instrument, the sodium recovery is reduced by a moderately high (70-130 IU ml-1) concentration of heparin or by the concomitant addition of the sodium ion which increases the ionic strength of the sample. The effect of the increased sodium chloride was studied separately, adding pooled sera to tubes containing dried sodium chloride in increasing quantity. The sodium recovery was reduced when the ionic strength of the sample was increased. We were unable to separate the negative effects on the sodium recovery due to heparin and due to the increased ionic strength in the sample.