Differences Between a Manual & Coulter HCT

Hematocrit, or HCT, represents the the percentage of red blood cells, by volume, in a human blood sample. Testing blood for HCT is important for medical conditions such as anemia. Manual and Coulter present two different testing methods to determine HCT. Differences exist between two methods, both in procedures and results.

Instrumentation

• Coulter refers to BeckmanCoulter, a company that manufactures medical equipment. One of their devices is the Coulter LH 750 Hematology Analyzer. This device does all the calculations required to obtain the HCT count in a blood sample. This method differs from the manual method which uses a blood sample that is placed into a tube and then subjected to 10,000 revolutions per minute (rpm) for five minutes. Known as a centrifuge, the method causes the red blood cells to settle down to the bottom of the tube with a layer of white blood cells on top followed by the blood plasma at the top of the tube. By measuring the height of the red blood cells as a percentage of the total height of the sample, HCT is determined. This is accomplished by visual inspection, hence the manual designation.

Accuracy

• Coulter HCT depends entirely on the analyzer for accuracy. According to BeckmanCoulter, the machine has the latest technology and produces the highest degree of accuracy. A question and answer about HCT testing in the American College of Pathologists pointed out how switching from Coulter to manual for one patient over a period of time can produce different numbers, thus causing problems. Coulter produced numbers between 45 and 50 for several tests while the manual test taken later showed a 56. There are some problems with the manual count due to trapped plasma in the red blood cells caused by some disorders. This trapped plasma raises the HCT above that reported by Coulter.

Human Factor

• Because the manual method requires a visual inspection while Coulter relies totally on automated equipment, the human factor in the manual calculations provides a difference. An article in the College of American Pathologists calls manual HCT testing the "gold standard", a reference to it being considered the better test by the author.