Secondary haemostasis and laboratory tests

Platelet count checks number. Buccal mucosal bleeding time (BMBT) screens platelets, vWF and the vessel wall. Von Willebrand disease (vWD) may have a normal count but prolonged BMBT. Device and method matter. My dog 4-minute/cat 2-minute cues differ from this edition’s dog <3 minutes/sedated cat 34–105 seconds.

The cascade helps read plasma tests. aPTT covers intrinsic and common pathways; PT covers extrinsic and common. VIIIa works with IXa before X, despite its low position in my sketch. VIII deficiency causes haemophilia A; IX deficiency causes B. vWF carries and protects VIII.

In living blood, Tissue-factor-bearing cells make a little thrombin. It activates platelets and cofactors. Complexes on activated platelets then make a thrombin burst and fibrin. Initiation, amplification and propagation overlap.

Vitamin K supports II, VII, IX and X. Warfarin blocks vitamin K epoxide reductase. PT often prolongs first; the book gives VII a 4–6-hour half-life. Disseminated intravascular coagulation (DIC), hepatic failure and unfractionated heparin can prolong aPTT. The 25–30% cue is NORMAL REMAINING in an isolated factor defect, with reagent limits.

ACT uses whole blood and contact activators, including glass, to trigger XII. It is less sensitive than aPTT; platelets and anaemia affect it. XII deficiency can prolong tests without a bleeding tendency.

Thrombin clot time adds thrombin to test fibrinogen-to-fibrin conversion. Low/abnormal fibrinogen and inhibitors prolong it. The book’s <50–100 mg/dL describes severe depletion before routine times often prolong, not a universal TCT cutoff. One sketch appears to say mg/µL; the paired sketch and book use mg/dL. The original/book “hetastarch: most pronounced dilution” cue is context-dependent; effects vary with formulation and setting.

Read tests with the patient. They do not reliably predict surgical bleeding.

References

Johnston/Tobias 2nd ed., Ch.7; Alatzas 2014; Cornell eClinpath.