Notebook: Hemostasis -- Coagulation

The following notes were taken during a 2-hour lecture by Dr. Urs Giger, DipACVIM.

Brief introductory discussion on the molecular level:
- activation of protein C.
- in order to be functional, protein C has a cofactor (protein S)
- both protein C and protein S are vitamin-K-dependent factors.
- complex of protein C and S will inhibit factors 5 and 7
- complex will also activate the inhibitor of plasminogen activator.
- this is a very regulated system that has to be balanced.
- Imbalances will lead to either a tendency to bleed or a tendency to thombose.

When an animal has a clotting problem, we need to know if the problem is primary or secondary. Problems may be acquired (any time, any age) or hereditary (for example, von Willebrand disease and hemophilia).

When an animal presents, ask the owner if there is a possibility of rodenticide problem.

During the physical exam, characterize bleeding.

COAGULOPATHIES: Problems with Secondary Hemostasis
- use a diagnostic test to assess hemostasis
- cuticle bleeding time: assesses overall hemostasis. Not standardized, painful.
- this method doesn't differentiate between problems with primary and secondary.
- "an absolute must" for every patient: blood smear evaluation
- look for platelets; if absent, ask yourself: is this a true thrombocytopenia?

Additional diagnostic testing
- do this prior to treatment (most situations -- unless patient is critically ill)
- why? b/c any treatment you do may change these results.
- Citrate tube: Tube contains 3.2% citrate solution that is added to the blood.
- Remember, without calcium, there is no coagulation
- The citrate is in a molar form, so calcium binding is a reversible process.
- (An EDTA sample chelates calcium and therefore is irreversible.)
- To perform the test, the tube must contain 9 parts blood and 1 part citrate.

- Collection technique is very important:
- if technique is incorrect, you may activate the coagulation pathway
- this may leave to shortened coagulation time.

- Activated Coagulation Time
- uses surface activation
- does not involve extrinsic clotting pathway
- does not require instrumentation
- does not require anything but the tube containing diatomaceous earth
- use vacuutainer; once first drops of blood are in the tube, start the timer
- when it is filled, pull it off; make sure there is good hemostasis
- mix by gently rotating the tube in your hands
- results: if there are no platelets, the actual clotting time will be prolonged.

[2nd HOUR OF LECTURE]

Partial Thromboplastin Time (laboratory PTT/aPTT)
- reactants: calcium, PTT activator (phospholipid), plasma
- phospholipids need to be added b/c plasma lacks platelets
- results are given in unit time, not in percent or in amount of protein
- in a way, it's assessing the same systems as ACT (intrinsic & common pathways)

Prothrombin Time (laboratory PT)
- assesses the extrinsic and common pathways
- reactants: calcium, tissue factor, plasma
- assay can be done within an hour; results are very reliable.

Protein Induced by Vitamin K Antagonism or Absence (PIVKA test)
- important for the main reason of anticoagulative rodenticides
- similar or identical to PT
- it is NOT specific for rodenticide poisoning.
- no reason to do this anymore. (PT test gives the same information)
- note: PTT test is most useful.

Fibrin degradation
- two ways to estimate fibrinogen:
- Thrombin Time (TT)
- not affected by rodenticides
- Plasma Fibrinogen Quantity

Thromboelastography
- overall hemostatic evaluation
- can predict thrombotic tendency
- can suggest fibrinolytic activity

Extra notes on hemostasis:
- Heparin is a very good coagulation inhibitor.
- Remember, you CANNOT DO coagulation tests with serum.
- The difference between serum and plasma: COAGULATION FACTORS -- mainly FIBRINOGEN.
- Schistocytes (spelling?) are seen when there are fibrin strands in a blood vessel.

Note to self: See Dr. Giger's lecture handout and complete the table on the second to last page.

TRANSFUSION MEDICINE
- indication for transfusion: anemia
- plasma may need to be given, or a combination of plasma and RBCs

Blood types are genetic markers on RBC surfaces; these are species specific and may differ between individuals.

Note to self: See more info on transfusion medicine in Dr. Giger's lecture handout.