Coagulation Testing

Question 1

What does aPTT and PT stand for?

Answer 1

PT: prothrombin time, usually expressed in INR
aPPT: stands for activated partial thromboplastin time

Question 2

All reactions that generate a vitamin K-dependent protease require ______ and _______

Answer 2

Ca++ and phospholipid (which is why testing tubules contain citrate, a weak sequestered of Ca++)

Question 3

Describe the components of the prothrombin test and what it tests for.

Answer 3

adding serum to thrombplastin (excess of TF, phopholipid and calcium) will result in bypass of amplification step by VIII and IX

test is sensitive for VII, X, V, II and fibrinogen

it is more sensitive than aPTT to warfarin’s effect

Question 4

What are the best clinical applications of PT?

Answer 4

best test of the integrity of the fibrin clotting system, defects the most common inherited clotting factor deficiencies

routinely used to monitor warfarin therapy but is insensitive to heparin at therapeutic concentrations

Question 5

Describe the components of the aPTT and describe which enzymes it is sensitive to.

Answer 5

adding citrated plasma with phospholipid contact system activator then adding calcium to allow clotting to complete to completion

sensitive to levels of contact factors, Xi, IX, VIII and all of the common pathway factors (not VII)
more sensitive to heparin than PT

Question 6

What are the clinical applications of PTT?

Answer 6

screening for inherited clotting factor
monitor heparin therapy
screening for acquired coagulation inhibitors (ie. factor VIII antibody)
a long PTT does not always indicate a bleeding disorder (as in case of contact factors or lupus anticoagulant)

Question 7

What causes a long PT and a normal PTT?

Answer 7

factor VII deficiency

mild deficiency of common pathway factors (warfarin, vitamin K deficiency and liver disease)

Question 8

What causes a long PTT and a normal PT

Answer 8

deficiency of VIII, IX, XI
deficiency of contact factor 
heparin (not typically LMWH
factor VIII inhibitor
lupus type inhibitor

Question 9

What can cause both PT and PTT to be exceptionally long?

Answer 9

liver disease
vitamin K difficiency
warfarin
DIC
high levels of heparin
inhibitor of common pathway (ie. direct thrombin inhibitor)
isolated deficiency of X,V, II or fibrinogen

Question 10

When is a thrombin time helpful?

Answer 10

very sensitive to heparin and other thrombin inhibitors; tod detect the presence of drugs like dabigatran

Question 11

When is a mixing study helpful?

Answer 11

used to determine whether a long aPTT or PT is due to clotting factor deficiency or circulating inhibitor; failure to correct after the addition of normal plasma, signifies the presence of circulating inhibitor

Question 12

What is the significance of a platelet function analysis?

Answer 12

measures how long it takes flowing blood to clot in collagen coated tube in presence of ADP or epinephrine which measures defects in platelet function, vWF activity

Question 13

How can we assess fibrinolytic activity?

Answer 13

by measuring blood levels of the products of fibrinolysis ie. D-dimer (elevated in DIC)
by measuring blood levels of fibrinolytic inhibitors alpha1-antiplasmin (consumption-note can also be low due to lack of production as in liver disease)

Question 14

If a patient presented with abnormal platelet function and a longer aPPT, what might you suspect if the PT was normal?

Answer 14

think vWF deficiency, and since FVIII carried on vWF, the aPPT is elongated

Question 15

In the case of isolated thrombocytopenia what is the most likely cause?

Answer 15

ITP, caused by autoimmune destruction of platelets by macrophages in the spleen and elsewhere (antibodies are IgG- can cross the placenta)

Question 16

What arm of the coagulation cascade are the aPTT and the PT sensitive?

Answer 16

aPTT: intrinsic IX or XI or common pathway
PT: extrinsic VII + common pathway

Question 17

____ can be somewhat elevated in a non-specific way in the postoperative setting.

Answer 17

D-dimer

Question 18

What treatments might you consider for a patient that has DIC and is bleeding?

Answer 18

admin FFP and cryoprecipitate (enriched in fibrinogen (antithrombin and antiplasmin levels reflect accelerated consumption of inhibitors)

Question 19

In a bleeding disorder that cannot be fixed by mixing study, what is the likely cause

Answer 19

a inhibiting factor, likely factor VII (can confirm with low VII levels)

Question 20

What are the two recessive inherited conditions that you should think about in bleeding disorders?

Answer 20

Bernard Soulier syndrome (ability to bind to vWF, associated with thrombocytopenia) and Glanzmann thrombasthenia (defective aggregation)

Question 21

Why should you omit the aPTT during routine pre-op screening unless the patient’s history and or family history suggest an underlying bleeding disorder?

Answer 21

an abnormal aPTT in an asymptomatic patient who has no family history of excessive bleeding is very rarely an indication of a bleeding tendency