Tuesday, July 30, 2019

Kleihauer–Betke test

Kleihauer–Betke test: KB test.

1) Why do we do this test?

- To calculate Fetal RBCs in blood. This helps us to measure amount of Anti-D required to neutralize it.

2) How do we do it?

- Basically, we are going to take blood sample and add acid to it and measure red blood cells under microscope.

3) How do you differentiate Fetal and Maternal blood?

- Fetal RBCs are acid resistant. Adding acid in the preparation leads to lysis of the Maternal RBCs.

4) What are important points regarding this test that should be kept in mind while solving MCQs?

- Do not confuse it with APT test. APT is done in Alkali and it is a Qualitative test. It helps in differentiating Maternal and Fetal blood only. On the other hand, in KB test (Also know as Acid dilution test), we use Acid and we quantify Fetal blood.

- Minimum dose even after KB test is 300 microgram.

5) How do we calculate amount of Anti-D required to neutralize Fetal RBCs?

- If 20 RBCs in HPF are seen, then it means 1 ml Fetal blood is in circulation.

-‎1 ml fetal blood requires 10 microgram of Anti-D for neutralization

6) What if they don't mention "Fetal RBCs" and instead, mention "Fetal blood" in the question?

- Here is a trick. Always remember, 1 ml Fetal blood has 0.5 ml Fetal RBCs.

Applied calculations:

Q1) A Multigravida with twin pregnancy has 20 ml Fetal RBCs. How much Anti-D will be required to neutralize it?

(Take a deep breath. You don't need to worry about twin pregnancy. All the important points are already covered in above segment)

- 20 ml Fetal RBCs = 40 ml Fetal blood.
- ‎1 ml Fetal blood = 10 micrograms Anti-D

Answer = 400 micrograms Anti-D

400 micrograms is the enough amount of blood given to neutralize 40 ml fetal blood or 20 ml Fetal RBCs.

(Done easily? Perfect ! Let's level up.
I want you to go through blog once again before heading down.)

Q2) This time patient comes with same clinical presentation but with 20 ml fetal blood.

- 1 ml fetal blood = 10 micrograms of the Anti-D
- ‎20 ml Fetal blood will require 200 micrograms Anti-D.

Perfect. We calculated correctly but my question is - Will you administer 200 micrograms Anti-D to the patient showing 20 ml Fetal blood to neutralize it?

Answer is big 'NO'.

Go back to bullet (4) point 2:

Minimum amount is still 300 micrograms after KB test. So you cannot administer 200 micrograms. You have to give 300 micrograms.

I hope this blog is better than my previous blogs. Any important points you have regarding KB test, do comment in comment box

That's it

-Demotional bloke


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