THE I-STAT ANALYZER, A NEW, HAND-HELD WHO LE-BLOOD ANALYZER FOR MEASUREMENT OF HEMATOCRIT, ELECTROLYTES, AND BLOOD-GASES

Exact and quick measurements of basic laboratory parameters are import ant in select ed patients in the perioperative period. Depending on th e capabilities of a hospital's central laboratory, the anaesthesiologi st may only obtain such laboratory tests after unacceptable delays. Th is problem may be overcome by a new bedside measurement device that ha s become available from i-STAT Corporation, Princeton, USA. The hand-h eld, battery-driven analyser accepts blood specimens that are injected into a disposable cartridge (EG7+) and measures acidity, blood gas te nsions, haematocrit, and electrolytes. The aim of this study was to de termine the accuracy of such measurements by comparing them with measu rements obtained by conventional laboratory test methods. Methods: Hep arinised arterial blood specimens were collected in duplicate from 49 surgical patients. Measurements of ionised calcium (Ca), sodium (Na), potassium (K), pH, pCO(2), pO(2), base excess (BE), haematocrit (Hct), and haemoglobin (Hb) obtained by the i-STAT analyser were compared wit h measurements from the calibrated analysers ABL 615 and EML 100 (Radi ometer, Copenhagen). Because the i-STAT analyser calculates the Hb con centration from a conductometrically measured Hct, 19 blood specimens were centrifuged in order to compare test results with conventionally obtained Hct and Hb values. As the Her test sensitivity with the i-STA T changes with diluted blood due to its low albumin concentration, Hct and Hb measurements during cardio-pulmonary bypass (CPB) must be corr ected by activating an analyser-implemented correction algorithm (Hct/ CPB and Hb/CPB). Correlation analysis was performed between convention al measurements and i-STAT values (Ca, Na, K, Hct, pCO(2), pO(2)), bet ween values that the i-STAT analyser derives (Hb, HCO3, BE) and conven tionally obtained results, and between normal and CPB-corrected Hct an d Hb values. Accuracy was judged according to the national quality sta ndard, which requires test results to lie within the 95% confidence in terval of conventional tests. Results: Each blood specimen was analyse d: erroneous results or technical failures did not occur. Measurement of one set of i-STAT values required 2.5 min. Correlation coefficients (r) between conventional and i-STAT results were: 0.85 for CA, 1.0 fo r K; 0.86 for Na; 0.99 for pH; 0.98 for pCO(2); 0.99 for pO(2); 0.93 f or HCO3; 0.93 for BE; 0.46 for Hb values not corrected for CPB and 0.9 5 for CPB-corrected Hb;and 0.74 for Her values not corrected for CPB a nd 0.98 for CPB-corrected Hct. The correlation coefficient for Hct bet ween centrifuged and CPB-uncorrected i-STAT values was 0.81 and that f or CPB-corrected values was 0.98. National accuracy requirements were not met for tests of: Ca (by 0.02 mmol/l); pH (by 0.01); pO(2) includi ng hyperoxic values(by 26.7 mmHg, but were met for pO(2) values <200 m mHg); Hb (by 1.6 g/dl); Hb/CPB (by 0.8 g/dl); and Hct (by 6.5%, but we re met for Hct/CPB values). All other tests fulfilled the required sta ndards. Conclusion: This analyser is easy to use, reliable, and portab le, and therefore suitable for the operating room, for analyses during emergencies, on peripheral wards, for preclinical screening, or at ti mes when availability of lab tests is time-consuming or limited. The t est accuracy for electrolytes, blood gases, and Hb is high enough to j ustify routine use of the i-STAT analyser in clinical practice. That t he nationally required quality standards for Ca, pH, and Hb were not m et is not of importance because the measured deviation was too small t o have clinical relevance. When analysing diluted blood with a low Hct and low oncotic pressure, it is important to activate the analyser's correction algorithm ''CPB'', because the obtained results will then c omply with the required accuracy.