Multicenter comparison of three intraoperative hemoglobin trend monitoring methods

Intraoperative Hb trend accuracy limits of agreement were approximately ± 1 g/dl for ABGHb and aHQHb and slightly larger for SpHb trends. Considering the 95% confidence intervals for agreement in change direction overlapped, our findings suggest that changes more than ± 0.5 g/dl in SpHb, ABGHb and aHQHb provide similar information regarding the direction of intraoperative tHb change, but not necessarily the magnitude of that change. The impact of trend accuracy on transfusion decision-making is more significant when tHb is less than 9.0 g/dl as current guidelines suggest that red blood cell transfusion may be reasonable [ 14] for tHb < 8.0 g/dl, but usually unnecessary [ 9] for tHb > 10.0 g/dl. Analysis of change direction agreement suggests that if continuous SpHb is < 9 g/dl then a decrease more than − 0.5 g/dl could be a good indication to obtain a blood sample for tHb measurement.

Repeatability analysis demonstrated limits of agreement that have implications for Hb accuracy and trend monitoring studies. The exclusion zones we defined based on 95% limits of agreement for ABGHb and aHQHB suggest that in our patients, a change up to ± 0.5 g/dl may not reflect a real change in circulating hemoglobin measured by tHb. Similarly, reported tHb changes of ± 0.25 g/dl may not represent real changes in circulating Hb. Our findings support use of an exclusion zone when comparing Hb analyzer trend performance and highlight the need to provide specific education regarding the exclusion zone appropriate to clinical use of any Hb monitor. This exclusion zone could be different in other centers or when using varying analyzers.

The SpHb results can be compared to prior reports of single center studies. A study of volunteers found 95.4% SpHb change agreement in 22 samples with tHb < 10.0 g/dl [ 15], which is similar to what we found. Using venous blood samples, a study of 70 trauma patients reported similar bias with narrower limits of agreement (− 0.05; − 0.62 to 0.51 g/dl) of SpHb change to tHb change than we report [ 16]. The number of patients and change calculations is smaller than included herein and reasons for the narrower limits of agreement are not clear, but may be partially explained by the multicenter source of our data. Studies of 48 vascular surgery patients [ 17] and of 70 patients undergoing major orthopedic surgery [ 18] concluded SpHb had clinically acceptable trending with ABGHb or tHb. In 49 patients undergoing spine surgery, change concordance was 85.1% when excluding SpHb with perfusion index under 1 [ 19]. When using the exclusion zones determined by measurement repeatability analysis but not excluding SpHb based on low perfusion index, SpHb concordance was 94.2%. Our findings are also better than reported in 69 patients undergoing spine or cancer surgery that employed a 1 g/dl exclusion zone [ 20]. A study of patients undergoing abdominal or pelvic surgery found 41 of 269 changes at any Hb had increased SpHb when ABGHb decreased, including 14 in which the decrease was more than − 1 g/dl [ 11]. Our finding of better trend direction agreement may be related to use of a newer SpHb version in the present study. We also found that Hb accuracy is improved compared to reports using earlier versions of the SpHb monitor [ 6– 8, 11].

Study limitations include having only about 10% of change samples with tHb ≤ 8.0 g/dl (43 of 416), which impacts our ability to assess clinical utility at very low tHb. However, change direction agreement was good for tHb < 9.0 g/dl. The number of samples in which SpHb, ABGHb or aHQHb change were not more than ± 0.5 g/dl represents a potentially problematic grey zone for assessing Hb change. Although at tHb < 9.0 g/dl only 1 ABGHb and 3 SpHb decreases in this grey zone had associated tHb changes more than − 1.0 g/dl, potential clinical impact of these is not clear and could be evaluated in a future prospective study. We studied SpHb Rev K but newer SpHb versions could potentially have different or better trend accuracy. This could be assessed in a future study. The use of arterial blood can be seen as problematic for generalization to clinical settings in which venous or capillary blood is more commonly sampled. We used arterial blood samples as we studied patients at risk for blood loss and routinely use arterial catheters in such patients to facilitate care and blood sampling. Although venous blood may be easier to obtain in some clinical situations, setting SpHb monitors to arterial mode and using only arterial blood removed potential confounding that a mix of arterial and venous blood samples could have introduced into trend accuracy analyses. Differences between arterial and venous Hb have been reported as 0.2–0.3 g/dl [ 21– 23]. We would expect trend calculations to be similar to our findings if only venous blood samples were tested. Use of arterial blood samples to determine aHQHb likely provides different results compared to capillary finger stick blood samples. However, our patients had as many as 13 samples which would have required many finger or ear lobe punctures to allow comparison to capillary samples, thus we chose to use arterial blood samples. Compared to our results, wider limits of agreement between tHb and point of care Hb have been reported when using capillary finger or toe stick samples in patients undergoing cesarean section [ 24], with gastrointestinal bleeding [ 25], or in the emergency room [ 26] and correlated poorly to arterial and venous tHb in intensive care patients [ 27]. Finally, we did not ascertain whether blood loss was suspected by clinical conditions but the range of tHb change in sequential samples was − 5.0 to + 3.7 g/dl, which reflects blood loss as well as transfusion decisions made by the anesthesiologists caring for the patients.

Hb measurement is essential to making patient centered transfusion decisions [ 28] which should improve outcomes [ 29] and reduce transfusion requirements [ 30– 32]. Hb measurement is particularly important when blood loss is not obvious or is difficult to estimate during surgery as blood sampling to determine tHb can lag clinical situations. Surgical patients are reported to receive both unnecessary or excessive transfusion [ 33, 34] and anemia and transfusion can increase perioperative morbidity and mortality [ 35– 37] for most but not all patients [ 38, 39]. However, postoperative outcome is reportedly better using less restrictive transfusion practices following some types of surgery [ 40], in elderly patients [ 41, 42] or in patients with cardiovascular disease [ 43, 44].