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.