Anomalies in pH 7.40 correction in ionised calcium analysers.
As a preliminary step in a study of the effects of calcium ligands on the pH standardisation of ionised calcium (Ca2+) measurement in blood, the change in Ca2+ induced by Pco2 variation was investigated in 12 serum pools on three different instruments. This type of study should yield a log Ca2+ = f(pH) linear relationship in a pH range around pH 7.40 with a slope characterising the pH-sensitive calcium buffer capacity of the specimen. The pH 7.40 correction line should be horizontal. This was the case for an ICA2 analyser but not for an ICA1 or a Nova 8 analyser. The difference was due to an incorrect setting of the built-in slope correction factor in the ICA2: fortuitously its value was close to the effective slopes of the serum pools used for the test. Thus the anomalous behaviour of the ICA1 and the Nova 8 was due to a discrepancy between the standard built-in algorithm and the characteristics of our serum pools. These findings led us to question the use of a constant correction factor to normalise actual ionised calcium values.
Sachs
C
C
Department of Physiology, University Paris V, Necker-Enfants Malades Medical School, France.
Chaneac
M
M
Rabouine
P
P
Kindermans
C
C
Dechaux
M
M
D001322:Autoanalysis; D002118:Calcium / Q000097:blood*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D012016:Reference Values
D002118:Calcium