Which of the following is the best method to ensure the correct amount of blood is introduced into microhematocrit tubes during sample preparation?

• A. Allow the tube to fill by capillary action until it is approximately two-thirds to three-quarters full.
• B. Use a pipette to add a random amount of blood to the tube.
• C. Fill the tube completely to the top to maximize sample volume.
• D. Fill the tube only halfway to prevent overflow.

Answer: (A)

Verified step by step guidance

1. Understand the purpose of microhematocrit tubes: they are used to measure the packed cell volume (hematocrit) by centrifuging a blood sample in a narrow tube.
2. Recognize that the volume of blood in the tube must be consistent and appropriate to ensure accurate and reproducible results during centrifugation.
3. Recall that microhematocrit tubes are designed to fill by capillary action, which naturally draws blood into the tube without the need for external force or measurement tools.
4. Identify that filling the tube approximately two-thirds to three-quarters full by capillary action ensures enough sample volume for proper centrifugation while avoiding overfilling, which can cause sample loss or inaccurate readings.
5. Conclude that using a pipette to add a random amount, filling completely to the top, or filling only halfway are not ideal because they either risk inconsistent volumes or insufficient sample for accurate hematocrit measurement.