The term hematocrit means "to separate blood. If the RBC and the hemoglobin are both normal, it is possible to estimate the hematocrit as being approximately three times the hemoglobin. Because the hematocrit is a percentage of red blood cells, as compared to the total volume of blood, any increase or decrease in plasma volume affects the hematocrit.
A patient with a severe burn loses huge amounts of plasma from damaged capillaries. As a result, the vascular space loses fluid in relation to cellular elements and the patient's hematocrit will be significantly increased. You have to carefully consider terminology and methods to answer the question above. Spun HCT and Spun PCV are different names for the same thing both are derived by the same method—determined by centrifugation of anticoagulated whole blood.
Calculated HCT is determined by automated hematology instruments impedance cell counters. A standard complete blood count is performed on an automated laboratory instrument that quantitates the amount of Hgb as well as the size, shape, and number of RBCs. A variety of calculations are performed to produce indices that provide information about RBC disorders.
The standard indices are:. The unit is a femtoliter fL , or it can be given as cubic millimeters mm 3. This value, along with Hct, provides the most useful measure of the oxygen carrying capacity of the blood.
Three factors besides the number of RBCs are critical for the determination of the hematocrit value when using a method based on measurement of electrical conductivity: Electrolytes Temperature Proteins Most blood gas analyzers allow for these variables as follows: Concentration of electrolytes: This is determined by one or more ion measurements.
As sodium is the primary electrolyte in plasma, this is the most important ion to measure and use in the calculation of hematocrit.
A change in the temperature has a significant impact on the conductivity because blood has a high temperature coefficient. The measuring chamber in blood gas analyzers is thermostatted and the blood sample preheated prior to measurement; thus there is no contribution from changing temperature.
The protein concentration in plasma is assumed constant in healthy people, so a constant compensation for this is incorporated in the calculation of hematocrit on blood gas analyzers. Limitations: In patients with abnormal plasma osmolality, e. Some blood gas analyzers offer correction for this bias [10,11,12]. Calculation of hematocrit from hemoglobin As there is a linear relationship between hemoglobin c tHb and hematocrit as described earlier, it is possible to calculate the hematocrit on analyzers that measure hemoglobin.
When making this conversion, two factors should be taken into consideration: The analytical quality of the c tHb measurement The precision of the equation that converts the two parameters The measurement of c tHb from most good-quality analyzers is usually reliable; however, the equations used to calculate the hematocrit vary from analyzer to analyzer.
Hematocrit and hemoglobin are often used interchangeably; however, different studies have shown that the two parameters are not comparable, but that they have their separate applications [15,17,18,19]. Comparing measuring technologies All measuring technologies for determining hematocrit have advantages and disadvantages. Table as word document. Method Advantages Microhematocrit Small sample volume Relatively fast analysis Hemolysis detected when result is read No dilution needed Complete blood cell count Hct parameter together with other hematology parameters — only one blood sample is necessary Dilution of sample removes problems with hyperosmotic samples No preparation needed No manual dilution needed Conductivity Small sample volume Short turnaround time Hct parameter together with blood gas, pH, electrolytes, and metabolites — only one blood sample is necessary No preparation needed No dilution needed Suitable for POCT Calculation from c tHb Hct parameter together with other parameters — only one blood sample is necessary Some methods are suitable for POCT TABLE IIa.
Advantages of different methods of measuring hematocrit. Method Disadvantages Microhematocrit Time-consuming and careful preparation required sealing of capillaries, etc Uncertain manual reading of the ratio Leakage of sealing gives falsely low results more RBCs will be lost than plasma [5] Falsely high Hct readings caused by trapped plasma.
Arterial blood has approx. Disadvantages of different methods of measuring hematocrit. Discussion When hematocrit is used to assess anemia and the oxygen-carrying capacity, the advantages and disadvantages of each method must be carefully considered.
Conclusion Both POCT and traditional laboratory methods of measuring or calculating hematocrit have limitations. Biochemical Basis of Pediatric Disease. Second ed. Kokholm G. Simultaneous measurements of blood pH, p CO 2 , p O 2 and concentrations of hemoglobin and its derivatives — a multicenter study. Radiometer publication AS Procedure for determining packed cell volume by the hematocrit method. Third edition. Fourth edition.
Louis, Missouri: Mosby, Hematocrit HCT. In: Thomas L, ed. Clinical laboratory diagnostics, use and assessment of clinical laboratory results. Known interfering substances.
Code no. Analytical artifacts in hematocrit measurements by whole-blood chemistry analyzers. Clin Chem ; 41, 2: User-defined corrections. Krarup T, Walcker KS. Guideline to measurement on the ABL using the user-specified fluid mode. Radiometer publication Bulletin No. Deriving a useful packed cell volume estimate from haemoglobin analysis. J Clin Pathol ; Clin Lab Haem ; Evaluation of automated methods of measuring hemoglobin and hematocrit in horses.
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