![]() Both optical and impedance assays can be affected by abnormal cell size and autoagglutination ( 10).Īlthough evaluated in human medicine, few studies in veterinary medicine have assessed the limits of agreement (LOA) between different Hct assays. Impedance measures the number of changes in electrical resistance passing through a specific orifice to determine the number of red blood cells ( 9). Optical measurement uses light transmittance and the absorbance spectra of red blood cells to determine the hematocrit ( 1). Therefore, when factors that affect conductivity (sodium, chloride, albumin, white blood cells, lipemia, anticoagulant and intravenous fluid therapy) become abnormal, the Hct measured may be inaccurate ( 3– 8). Conductivity measures the difference between the plasma, which transmits electricity, and the erythrocytes, which are non-conducting cells ( 1). ![]() It can be affected by excessive collection tube anticoagulant and improper sample preparation such as a prolonged delay between sampling and measurement, or extended storage times at room temperature ( 2). ![]() Centrifugation is considered the reference standard: often referred to as the microhematocrit or packed cell volume (PCV), it separates the cellular components of blood, allowing measurement of packed erythrocytes. There are four common assays used to measure blood hematocrit values: centrifugation, conductivity, optical reflectance and impedance ( 1). This creates a need to determine if Hct assays produce the same value in a variety of clinical settings, as an inaccurate result can affect clinical decisions and patient outcomes. Veterinary hospitals often use hematology devices interchangeably, even if the devices use different methodologies. Veterinary patients with erythrocytosis or anemia often require serial measurement of their blood hematocrit (Hct) to optimize diagnostic and treatment options. The wide limits of agreement also demonstrates that care must be taken when making clinical decisions with different hematocrit methodologies. Therefore, the hematocrit methods cannot be used interchangeably. This study confirms that albumin values outside reference limits can affect the conductivity method and that hematocrit values obtained via conductivity, optical and impedance methods underestimate values obtained via centrifugation. The hematocrit difference between conductivity and centrifugation methods was statistically different for low (4%, 0–5%), within reference limits (3%, −5 to 8%), and high (2%, −2 to 5%) albumin values, respectively ( p = 0.02), with post-hoc analysis demonstrating that the difference occurred between the low and high albumin groups. The conductivity method underestimated hematocrit by 2.1 ± 2.9% (95% LOA −3.54 to 7.88), the optical method by 3.1 ± 3.6% (95% LOA −4.0 to 10.2), and the impedance method by 2.3 ± 3.7% (95% LOA −5 to 9.6) when compared to centrifuged hematocrit values. There was a statistically significant difference between centrifugation hematocrit values and values obtained via conductivity ( p < 0.0001), optical ( p < 0.0001), and impedance ( p = 0.0082) methods. Bland-Altman analysis determined bias (± SD) and 95% LOA between methods. Canine venous blood was collected from 74 clinical cases and measured by centrifugation ( n = 72), conductivity ( n = 73), impedance ( n = 24) and optical ( n = 50) methods. The effects of albumin, which are known to affect conductivity methods, have not been evaluated for limits of agreement (LOA) between hematocrit assays in small animals. Centrifugation (reference standard), conductivity, optical and impedance methods are often used interchangeably to measure hematocrit. 4Department of Emergency Care, Western Veterinary Specialist & Emergency Centre–VCA, Calgary, AB, CanadaĬlinical decisions are influenced by hematocrit values.3Department of Veterinary Clinical and Diagnostic Sciences, Faculty of Veterinary Medicine, University of Calgary, Calgary, AB, Canada.2Department of Emergency Care, Central Victoria Veterinary Hospital–VCA, Victoria, BC, Canada.1Department of Internal Medicine, Western Veterinary Specialist & Emergency Centre–VCA, Calgary, AB, Canada.Amelie Pare 1 * Laura Kippen 2 Catherine Wagg 3 Matt Longmore 4 Soren Boysen 3
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