Accuracy, precision and reliability of the i-stat haemoglobin measurement

Wai Leap Ng, Barry Slon, Kerry Gunn, George Fuge and Tim Short

Auckland City Hospital, Auckland, New Zealand

Background: Several studies have found the i-Stat (Abbott, Illinois, USA) point-of-care testing device to measure haemoglobin (Hb) concentration accurately1,2. Clinical experience and one published study suggest otherwise3, especially when a blood sample has diluted plasma proteins. We investigated the accuracy of the i-Stat Hb measurement in surgical patients with expected blood loss of > 25%.
Methods: After obtaining institutional ethical approval, 31 patients were recruited. Blood tests for i-Stat Hb (tested twice), laboratory Hb (Sysmex XE2100, Roche, Kobe, Japan), and total plasma proteins were obtained at the start of surgery (T=0) and one when an estimated 25% blood loss had occurred (T=1).
Results: The coefficient of variation of the paired i-Stat Hb estimates was 2.8% at T=0 and 2.9% at T=1. The mean difference between the i-Stat and laboratory Hb was -7.6 mmol/L (sd=5.6) at T=0 and -6.0 (sd 11.5) at T=1. The two methods gave significantly different Hb estimates (t-test, p<0.01). The total plasma protein was 64 mmol/L (sd 5.6) at T=0 and 50.6 (sd 9.2) at T=1 (a 20% decrease). The mean i-Stat Hb was 113 mmol/L at T=0 and 93 at T=1 (a 17% decrease). The correlation coefficient between i-Stat and total protein using combined T=0 and T=1 data was 0.52 (ns).
Conclusions: The i-Stat Hb had an acceptable coefficient of variation, but the Hb levels were significantly lower than those estimated by the laboratory. The standard deviation was greater after 25% estimated blood loss, there was no evidence that this was due to decreased plasma proteins. The clinical perception of inaccurate i-Stat Hb estimates may have been due to the increased standard deviation in Hb estimates after haemodilution.
References:
1 Bingham D, Kendall J, Clancy M. The portable laboratory: an evaluation of the accuracy and reproducibility of i-STAT. Ann Clin Biochem 1999;36:66-71.
2 Papadea C, Foster J, Grant S, Ballard SA, Cate JC, Southgate WM, Purohit DM. Evaluation of the i-STAT portable clinical analyzer for Point-of-Care blood testing in the intensive care units of a university children's hospital. Ann Clin & Laboratory Science 2002;32:231-243.
3 Hopfer SM, Nadeau FL, Sundra M, Makowski GS. Effect of protein on hemoglobin and hematocrit assays with a conductivity-based Point-of-Care testing device: comparison with optical methods. Ann Clin & Laboratory Science 2004;34:75-82.