Following on from parts one and two in this coagulation series – in which we described how to use your history, signalment and clinical signs to help determine if your patient has a coagulopathy and narrow down the list of differentials – we now look at the diagnostic tests you can use to confirm your suspicions.
In the initial stages, you only need two simple tests – a blood smear and an activated clotting time.
Blood smear – evaluates primary haemostasis (platelets)
- Less than between 35,000 platelets per μL and 50,000 platelets per μL – equivalent to less than two to three platelets per 100× field – is required for a spontaneous bleed and you should consider that your patient has a clinically relevant thrombocytopenia.
- If you see more than 10 platelets per 100× field, you do not have a thrombocytopenia causing bleeding.
Pro Tip #1
Anytime you are collecting blood, keep one drop for a smear – it saves you from having to stick the patient again, and you can easily throw it away later. You can also perform an ear prick if you don’t want to ruin a vein.
Pro Tip #2
If you can‘t find a platelet in the monolayer, check the feathered edge. If you see platelet clumps then you are all good.
If platelet numbers are normal then you either have:
- a platelet disorder (thrombocytopathia)
- a problem with secondary haemostasis (clotting factors)
Activated clotting time – tests secondary coagulopathies
Normal activated clotting time (ACT) is generally less than 120 seconds, or 90 seconds with some types of tubes. The issue with this test is that it requires a significant deficiency in clotting factors (less than 5% normal factor activity) for prolongation to occur. However, this is usually not a problem if they are clinically bleeding.
As a rough rule, if the ACT is greater than 25% of normal then it can be considered abnormal and you have a secondary coagulopathy. The ACT is an inexpensive and quick bedside test; however, partial thromboplastin time (PTT) and activated partial thromboplastin time (aPTT) are more sensitive, which will be further discussed in the next part.
Pro Tip #3
ACT can be prolonged with severe thrombocytopenia, so check your platelets.
Pro Tip #4
PT and aPTT are not affected by platelets like the ACT.
If your ACT is normal and your platelets are normal:
- Consider thrombocytopathia. However, testing to see if PTT and aPTT are normal as well helps rule out a secondary coagulopathy.
- Thrombocytopathia is assessed by performing a buccal mucosal bleeding time (BMBT), which will assess for platelet disorders like von Willebrand disease.
- A BMBT of more than four minutes is abnormal.
Pro Tip #5
Avoid using a scalpel blade for a BMBT due to a large amount of error. Use a specific “surgicut”.
In the next part, we delve deeper into secondary coagulation disorders.
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