HCT Procedure

Spinning And Reading A Manual Hematocrit (HCT)

Principle

Spinning an aliquot of blood in a microhematocrit glass capillary tube packs the red blood cells. The hematocrit expressed as the volume of packed red blood cells that occupies a specific volume of whole blood. Hematocrits are expressed in percentage (%) or L/L (used in Canada).

The relationship between the red blood cell population, hematocrit and hemoglobin is represented by the rule of three, where the hematocrit is approximately three times the hemoglobin value. These three RBC parameters are used to calculate the RBC indices that aid in the classification of anemia.

Materials

• Microhematocrit centrifuge
• Microhematocrit capillary tubes
• Microhematocrit reader
• Citroseal vinyl plastic putty
• EDTA or capillary blood sample
• Sample rack
• Gloves
• Biohazard Bucket

Sample Type

• EDTA anticoagulated blood sample, via venipuncture
• Fresh capillary blood can be used, finger or heel stick

Safety Considerations

1. Follow safety protocol for using a centrifuge.
2. All specimens received in the lab must be regarded as potentially infectious so follow safe practices.
3. Refer to regulations and safety precautions in the student lab manual located in each student lab.

Procedure

1. Mix the sample for 2 minutes. If the sample has been refrigerated, allow to sit for 15-20 before testing.
2. Record on the HCT Worksheet the patient’s first and last name, accession number and MRN.
3. Fill two capillary tubes about ¾ the way to the top with EDTA whole blood or capillary blood.
4. Wipe away the first drop of blood before sealing the tube.
5. Place the dry end of the capillary tubes into the “Citroseal” and seal the ends. Caution: These are glass capillary tubes so press down on them gently when sealing them.
6. Place both the capillary tubes in the centrifuge across from each other making sure they are balanced and that the sealed end is facing towards the outside of the centrifuge
7. Document the position of the tubes on the worksheet provided next to you already recorded patient information
8. Place the head cover on and lock it in place. This step is very important. If you forget the head cover the capillary tubes will break and blood will spill into the centrifuge and require cleaning.
9. Close the top cover and lock it by pushing in on the latch until you hear a click.
10. Set the centrifuge time for 5 minutes by turning the timing knob clockwise.
11. Wait for the centrifuge to come to a full stop before you open the top cover.
12. Unlock the inside head cover and carefully remove the capillary tubes.
13. Place them on the microhematocrit reader to read. Refer to appendix #1 for using the reader.

Procedural Notes

1. This centrifuge spins samples at 8000rpm’s for 5 minutes. Proper time and speed of centrifugation are important
2. If you use capillary blood, wipe away the first drop of blood to avoid interstitial fluid contamination.
3. Make sure the tube is sealed properly to avoid leaking.
4. Make sure you lock both the inside head cover and outside cover before spinning the blood.
5. We report hematocrits in L/L units in Canada.

Quality

Hematocrits are reported in L/L in Canada. You may still hear some use the older units (%). Percentage is used in the US.

Report the average of both reading and they must not vary by more than 5%.

Normal Values Vary with Age

• Newborn:        0.530L/L – 0.650L/L
• Adult male:     0.420L/L – 0.520L/L
• Adult female:  0.370L/L – 0.470L/L

Interpretation

Patient 1

HCT = 0.360L/L and HGB of 120g/L. Values conform to the rule of three.

Patient 2

HCT of 0.300 and HGB of 80g/L. Values do not conform to the rule of three.

If values do not agree, exam peripheral blood film for abnormal RBC populations.

Patients with lipemic plasma may lead to a false increased HGB value.

When unexplained values are identified the technologist should run the sample just before and after, the sample in question, to see if they conform to the rule, and if not, further investigation maybe required.

Sources of Error (manual Hct)

Falsely decrease values

1. If the capillary tube is not sealed properly
2. When increased anticoagulant concentration is present in the tube due to a short draw.
3. Improper use of the microhematocrit reader.
4. Specimen is not mixed properly
5. Over-spinning or higher than required centrifuge speed
6. Immediately following blood loss (temporary).
7. Contamination with interstitial fluids.

Falsely increased values

1. Insufficient spinning or delay in reading.
2. Including the buffy coat in the reading.
3. Improper use of the microhematocrit reader
4. Specimen not mixed properly.
5. Under spinning or lower than required centrifuge speed
6. Many anemia related disorders cause plasma to be trapped in the RBC layers
7. Fluid loss associated with dehydration.^[1]

Note: These sources of error are based on a manual spun hematocrit and may not be affected in samples run on an automated blood analyzer.

Using a microhematocrit reader

1. Place the capillary tubes in the groves on the plastic holder.
2. Line up the bottom of the red blood cells with the line on the holder.
3. Move the holder to the right until the top of the plasma sits on the 100% line above.
4. Move the reader bar up until the line on the bar intersects with the line separating the RBC’s from the plasma.
5. This will move the counter above. Read the % or packed red blood cells in the window at the top of the reader.
6. Document the results on the worksheet provided.
7. Repeat the procedure for the second capillary tube.
8. Take the average of both readings and report the result in L/L.