Coagulation Tests

Question 1

List 3 tests which measure primary haemostasis.

Answer 1

Bleeding time
Platelet count
Platelet function tests (light transmission aggregometry)

Question 2

What is the normal bleeding time?

Answer 2

3-8 minutes

Question 3

What is the normal platelet count?

Answer 3

150-400 x10^9/L

Question 4

List 4 tests which measure secondary haemostasis.

Answer 4

Prothrombin time (PT)
Activated partial thromboplastin time (APTT)
Thrombin clotting time (TCT)
Individual clotting factor assays

Question 5

What is the normal prothrombin time (PT)?

What is the normal PT ratio?

What does it measure?

Answer 5

10-13 seconds

Ratio: 1.0-1.2

Extrinsic pathway

Question 6

What other calculation can you do with the prothrombin time?

How do you calculate this?

Answer 6

International normalised ratio (INR)

INR = [prothrombin time ratio]^ISI

Where:
ISI = international senstivity index, different for each brand of thromboplastin
PT ratio = PT time/average of 20 PT results

Question 7

What is the normal APTT?

What does it measure?

Answer 7

26-38 seconds

Intrinsic pathway

Question 8

What other test might you do if the APTT is prolonged?

Answer 8

APTT 50:50 test - add normal plasma, then repeat test

If APTT corrected: suggests factor deficiency
If APTT only partially corrected: suggests inhibiting factor present, e.g. lupus anticoagulant

Question 9

What is the normal thrombin clotting time (TCT)?

What does it measure?

Answer 9

10-16 seconds

Fibrinogen levels/function

Question 10

Describe how to do a direct Coombs test. (3)

Answer 10

1. Patient’s blood is mixed with antihuman antibodies (i.e. antibodies against human Ig)
2. If the patient has autoimmune haemolytic anaemia, then there will be Ig bound to their RBCs
3. Antihuman antibodies will bind to Ig on the patient’s RBCs to give a positive Coombs test

Question 11

What does a positive direct Coombs test indicate?

Answer 11

Autoimmune haemolytic anaemia (AIHA)

Haemolytic disease of the newborn

Question 12

When would you do an indirect Coombs test?

Answer 12

Blood transfusions

Question 13

Describe how to do an indirect Coombs test. (4)

Answer 13

1. Patient’s blood is washed to remove all cells (so that only plasma remains)
2. Patient’s plasma is mixed with the blood that is about to be transfused into them
   a. If the patient’s blood contains antibodies, they will bind to the RBCs in the donor blood
3. Antihuman antibodies are added to the mixture
4. Antihuman antibodies will bind to the Ig on RBCs to give a positive Coombs test

Question 14

What does a positive indirect Coombs test indicate?

Answer 14

Incompatible blood transfusion