[Analytical performances of the i-STAT portable clinical analyzer for the measurement of PO2 and PCO2]. / Evaluation des performances analytiques du i-STAT portable clinical analyzer pour la détermination des pressions partielles d'oxygène et de gaz carbonique dans le sang total.

Aware of some limitations on blood gas results, we performed an extensive evaluation before introducing i-STATs in our hospitals. Three i-STATs were tested in parallel with an ABL-520, on three types of cartridges (EG7, EG6 and EG3), using tonometered whole blood (9 gas levels, n = 720) and aqueous QC solutions (3 levels, n = 600). Reference systems were the theoretical calculated values from gas composition used for tonometry and results given by the ABL-520, respectively. On aqueous controls dispersion intervals reached 10-20 mmHg for both analytes for inter-lot as well as intra-lot data. PO2 values on blood showed marked dispersion: 5 mmHg (CV = 2 to 7%) at clinically critical levels. PCO2 showed several (10%) major outliers: mV recording of the PCO2 electrode allowed to incriminate a pre-humidification problem (due to incorrect shipping conditions). Once outliers have been discarted, there still was a 5 mmHg non negligible residual dispersion (CV = 3 to 5%). i-STAT analytical performances for blood gases which are the analytes whose determination at the bed-side is potentially the most useful, do not match capabilities of classical laboratory instruments. Thus even though the i-STAT approach represents a seducive solution for the STAT problem, for the moment, it's use cannot be recommended in a hospital environment where classical instruments can be made available.

Evaluation of syringes for ionized calcium measurements.

UNLABELLED: Commercially available ready to use syringes (Radiometer, Sarstedt, Ciba-Corning and Portex), containing heparinate as an anticoagulant have been tested to evaluate the magnitude of induced preanalytical errors. Tonometered serum pools adjusted to four Ca2+ concentrations were sampled anaerobically. MEASUREMENTS: Ca2+ and pH (ICA2 with 3 digits, Radiometer Medical A/S, Denmark; Heparin: anti-Xa factor activity on a chromogenic substrate. Results were expressed as means of 10 measurements and as percentages of the reference values. Sarstedt syringes, (Li-heparinate), yielded a negative bias (-3%). However for 0.5 or 1 mL samples the bias reached -4% to -6%. Radiometer syringes (QS50 and QS90; calcium titrated heparinate) demonstrated biases below -2%. The bias in the Ciba-Corning (Gas-Lyte) syringe was below 2%. Portex (Pulsator) syringes showed biases above +4% even for nominal sampling volumes. All syringes (except Pulsator) released anticoagulant amounts corresponding to the expected values. Radiometer and Ciba-Corning were the only recommendable devices.

Evaluation of capillaries for ionized calcium measurements.

Commercially available capillaries containing calcium-titrated heparinate as an anticoagulant designed specifically for ionized calcium measurements in blood were tested with four serum pools with ionized calcium concentrations adjusted to 0.75, 1.25, 1.50 and 2.50 mmol/L. Although this is the best available anticoagulant for this purpose, the use of these capillaries induced a +/- 1% alteration of the original concentration around the titration level and changes of -2 to +10% at pathological values. The amount of heparinate released exceeded the recommended limit of 50 IU/mL of specimen with some variability (6% to 20%). Increasing the amount of anticoagulant with the objective of avoiding magnetic mixing did not seem to be a valid approach. Finally, Radiometer, IL and AVL capillaries gave the best available and acceptable results in terms of alterations in ionized calcium.

In vitro evaluation of a heparinized blood sampler for ionized calcium measurement.

When ionized calcium measurements are needed urgently blood has to be sampled with an anticoagulant to allow rapid specimen processing. Heparinate salts cause a decrease in ionized calcium by binding which is clinically significant when the concentration exceeds 15 IU/mL of whole blood. The use of an anticoagulant in an aqueous state induces 'solution-dilution' errors. To avoid these two types of error the use of calcium-titrated sodium heparinate in a dry state has been proposed. However, in this situation the actual calcium concentration could be modified if its value were different from the titration level. This possibility has been studied using a commercially available sampler (Radiometer B-129). When the sampler was used as recommended, errors were non-significant around 1.25 mmol/L. There was a +3% increase for low (0.75 mmol/L) values and a -2% decrease for high (2.50 mmol/L) values. Incomplete syringe filling increased these errors.

Use of calcium-titrated sodium heparinate for ionized calcium determination on plasma and whole blood (heparinized blood sampler B-129 Radiometer).

We report modifications of ionized calcium as observed on specimens collected in a commercially available sampling device (Radiometer B-129). When the sampler is used as recommended, errors are non-significant around 1.25 mmol l-1. There is a +3% increase for low (0.75 mmol l-1) values and a -2% decrease for high (2.50 mmol l-1) values. Incomplete syringe-filling increases these errors.

Preanalytical errors in ionized calcium measurements induced by the use of liquid heparin.

The use of heparin in a liquid form to measure ionized calcium (Ca++) in plasma or whole blood can induce preanalytical errors by dilution and by changing the original Ca++ value by binding or by re-equilibration with calcium in the anticoagulant solution. To quantify these errors, Ca++ was measured on serum pools under different sampling conditions. Incomplete syringe filling and specimen volume/syringe nominal volume ratio effects were tested. Syringes were rinsed with saline to yield pure dilution effects, with sodium heparinate to study binding and with calcium-titrated heparinate to evaluate 'calcium-distortion'. Detailed tables provide percentage error values for all sampling conditions. Dilution errors could reach -5% and binding was always important (-14 to -50%). Distortion was minimal around 1.25 mmol/L but could reach -4% for high and +8% for low Ca++ values. Errors increased when syringes were not filled to their nominal volume, especially with small-sized specimens.

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.

Anticoagulant-induced preanalytical errors in ionized calcium determination on blood.

When anticoagulated blood is necessary for ionized calcium (Ca2+) measurements especially in urgent circumstances, the type (sodium or 'calcium-titrated' heparinate) as well as the form (aqueous or dry) of anticoagulant induce preanalytical errors. To quantify these modifications Ca2+ was measured in three aqueous solutions and in three serum pools in different 'sampling' conditions. Incomplete syringe filling and specimen volume/syringe nominal volume ratio effects were tested. Syringes were rinsed (i) with saline to yield 'pure' dilution effect ('solution-dilution'); (ii) with sodium heparinate to study binding; (iii) with calcium-titrated heparinate to evaluate 'calcium-distortion'. All types of errors increased when syringes were not filled to their nominal volume, especially on small-sized specimens. Detailed tables provide percentage error values for all sampling conditions. Thus, 'solution-dilution' can reach -5%; binding is always important (-15 to -50%). 'Calcium-distortion' is minimal, around 1.25 mmol/l Ca2+, but can reach -7% for high Ca2+ and +10% for low Ca2+.

Inadequate algorithm: a cause for 'incorrect pH 7.40 correction' in ionized calcium analysers.

As a preliminary step in a study of the effects of calcium ligands on the pH standardization of ionized calcium (Ca2+) measurements in blood, the slope of logCa2+ = f(pH) linear relationship characterizing the pH-sensitive calcium buffer capacity of the specimen was investigated in 12 serum pools on three different instruments. The pH 7.40 correction line should be horizontal. This was the case for the ICA-2 but not for the ICA-1 and the NOVA-8. The discrepancy was caused by an incorrect setting of the built-in slope correction factor in the ICA-2; coincidentally, its value was close to the effective slopes of the serum pools used in the study. Thus, the 'abnormal' behaviour of the ICA-1 and the NOVA-8 was caused by an inadequacy of the built-in algorithm to the characteristics of our serum pools. These findings lead us to reconsider the use of a fixed and constant correction factor to normalize actual ionized calcium values.