Point of Care Testing for Blood Conservation Strategies: Finding the Right Tools Matters

Andrew D Jones, MD, MA, PGY2, Pathology and Laboratory Medicine
Fabian Lara, MD, Postdoctoral Scholar, Pathology and Laboratory Medicine
Nam K Tran, PhD, MS, FACB, Director of Clinical Chemistry and POCT

Background:

Patient Blood Management programs have, over the last several years, been working hard to reduce unnecessary red blood cell transfusions.  For most hospitalized adult patients, a cutoff of 7 g/dL is used to balance the risk of blood transfusions with the risk of decreased oxygen carrying capacity.1 Special populations such as cardiac or burn surgery patients, may benefit from different cutoff values.2,3 Point of care (POC) testing is often employed in perioperative settings quickly measure hemoglobin levels and guide transfusion practices.  There are different ways to measure hemoglobin in a sample, and not all instruments are equally efficacious. We summarize hemoglobin measurement methods below:

Spectrophotometric Techniques: Spectrophotometry is often used to measure hemoglobin and is often found in hospital laboratory settings and select POC devices. This is based on the light absorption spectra of oxyhemoglobin and deoxyhemoglobin which allows the device to measure the concentration of hemoglobin based on the amount of light that passes through a sample of a given size.  In these methods, red blood cells (RBC) may or may not be lysed, and the wavelengths of oxy- and deoxyhemoglobin can be measured and the concentration of hemoglobin can then be determined. This same technique has been applied to non-invasive CO-oximetry and enabling real-time trending of total hemoglobin levels.4 Other spectrophotometric-based techniques utilize the light absorption of cyanohemoglobin. The hemoglobin found in the patient sample is first converted to cyanohemoglobin using a solution of potassium cyanide and potassium ferricyanide.  Cyanohemoglobin can then be measured more specifically by these methods and is considered the “gold standard” for hemoglobinometry. However, it must be noted that cyanohemoglobin methods are not flawless and patients with sickle cell disease and other hemoglobin variants (i.e., fetal) can cause inaccurate results.

Conductance-Based Techniques: Conductance-based devices measure hematocrit and calculate hemoglobin. Due to the simplicity and convenience of these biosensors, several POC blood gas devices employ conductometry to calculate hemoglobin. These devices measure the conductance of RBCs between two electrodes.  In brief, red cell membranes are non-conductive, therefore an increase in the concentration of RBCs will result in a proportionate increase in resistance (and decreasing conductance). In effect, as hematocrit increases, conductance decreases (Figure 1).  Calculation of hemoglobin from hematocrit can be accomplished by multiplying by 0.34.5 This 0.34 conversion factor is acceptable in most populations, however some patients with red cell abnormalities (like patients with malaria) require a different conversion factor to accurately estimate the hemoglobin concentration when measured by this method.6 More common, patients experience hemodilution and resulting in hypoproteinemia may also cause inaccurate calculated hemoglobin results. Plasma proteins also influence conductance in these devices. In the case of hemodilution resulting in hypoproteinemia, falsely low hematocrit levels may arise on conductance-based devices and result in unnecessary transfusions.7

Figure 1: Conductance-Based Hemoglobin Sensors.

Conductance-Based Hemoglobin Sensors

Management:

At our institution, a study was conducted to evaluate the risk of unnecessary transfusion resulting from biased hemoglobin values secondary to hemodilution when using conduction impedance point of care hemoglobin measurements.  Sixty patients who were to undergo cardiac surgery had hemoglobin levels tested using the handheld POC conduction impedance method, in addition to the standard spectrophotometric method in the core laboratory.  The mean bias of the POC analyzers was -1.4 g/dL (p=0.011) (Figure 2).  Had these patients been tested using only the conductance-based POC analyzer, 12 of the 60 would have had hemoglobin values less than 7. In contrast, paired measurements made by spectrophotometric-based devices report results at or above the 7 g/dL transfusion threshold.  Thus, clinicians should be aware of their patient’s hemodilution status and the type of POC hemoglobin analysis being performed. Spectrophotometric-based hemoglobin testing methods are recommended for patients at risk for hemodilution.

Figure 2. Comparison of Conductance-based vs. Spectrophotometric-Based Hemoglobin Testing Methods. The figure is a Bland-Altman plot illustrating the analytical performance of a handheld conductance-based hemoglobin testing method when compared against a spectrophotometric-based (Reference, x-axis) method. The total hemoglobin bias (y-axis) is the POC device minus the paired result from the reference method. A hemoglobin transfusion threshold of 7 g/dL is used and the shaded area indicates patients that would have been inappropriately transfused if using the conductance-based results.

Comparison of Conductance-based vs. Spectrophotometric-Based Hemoglobin Testing Methods

References:

  1. Carsen JL, Guyatt G, Heddle NM, et al. Clinical Practice Guidelines From the AABB. JAMA 2016;316:2025.
  2. Palmieri TL, Caruso DM, Foster K, et al. Effect of blood transfusion on outcome after major burn injury: A multicenter study. Crit Care Med 2006;34:1602-607.
  3. Murphy GJ, Pike K, Rogers CA, et al. Liberal or restrictive transfusion after cardiac surgery. N Engl J Med 2015;372:997-1008.
  4. Kost GJ and Tran NK. Continuous noninvasive hemoglobin monitoring: the standard of care and future impact. Crit Care Med 2011;39:2369-371.
  5. Abbott i-STAT, Hematocrit/HCT and calculated hemoglobin/HB Product Insert. https://www.pointofcare.abbott/download?docUri=/technical-library/static-assets/technical-documentation/714178-00Q.pdf, Accessed on February 6, 2017.
  6. Rodríguez-Morales AJ, Sánchez E, Arria M, et al. Haemoglobin and haematocrit: the threefold conversion is also non valid for assessing anaemia in Plasmodium vivax malaria-endemic settings. Malaria Journal2007;6:166.
  7. Hopfer SM, Nadeau FL, Sundra M, et al. Effect of protein on hemoglobin and hematocrit assays with a conductivity-based point-of-care testing device: comparison with optical methods. Ann Clin Lab Sci 2004;34:75-82
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