Effect of protein on hemoglobin and hematocrit assays with a conductivity-based point-of-care testing device: comparison with optical methods.

Point-of-care testing (POCT) for blood hemoglobin and hematocrit (H/H) levels provides rapid patient assessment including the need for transfusion. Conductivity-based methods of blood H/H determinations can be influenced by plasma protein concentration. To assess this factor, we measured H/H levels at varying protein concentrations using two POCT instruments: iSTAT-1 (conductivity method) and Hemocue (optical method). These H/H results were compared to results obtained by our core laboratory hematology analyzer (GenS). Anticoagulated whole blood was centrifuged to sediment the red blood cells; the plasma was removed to serve as source of protein for mixing studies. A series of reconstituted samples was prepared with varying H/H and protein levels. To mimic hemodilution in clinical practice, samples were diluted with saline or lactated Ringer's solutions. Following H/H analysis, the samples were centrifuged and protein determined in the supernatant plasmas. The H/H results obtained with the Hemocue instrument correlated exactly with those of the GenS analyzer at protein concentrations from 0.7 to 6.2 g/dl. The correlation was unaffected when hemodilution was performed with either saline (r = 0.999) or lactated Ringer's (r = 1.000). The H/H results obtained with the iSTAT-1 instrument gave slightly less correlation with those of the GenS analyzer (r = 0.978 - 0.980) over this protein range. However, the iSTAT-1 results were generally lower than the GenS results, with discrepancies up to 2 g/dL for hemoglobin values and up to 4% for hematocrits at the lowest protein concentration. Therefore, it is recommended that H/H testing in patients with suspected hypoproteinemia or substantial hemodilution should be tested with a non-conductivity-based method.