Coagulation Testing

PT/APTT

	PT	APTT
Factors Affected	I, II, V, VII, X Most sensitive to VII (shortest half life)	All except VII - Congenital def. of VIII, IX, or XI - VWF disease → low VIII
Pathways Affected	Extrinsic and common	Intrinsic and common
Drugs Monitored	Coumadin (warfarin)	Heparin (unfractionated)
Conditions		Specific factor antibodies Non-specific inhibitors Coagulation factor deficiencies Unexplained bleeding/bruising Recurrent miscarriages Disseminated intravascular coagulation (DIC) Recurrent thrombosis Prior to surgery when increased risk of blood loss or history of bleeding
Interfering Factors		Drugs (antihistamines, ascorbic acid - Vitamin C, chlorpromazine, heparin, salicylates) Incorrect anticoagulant Hematocrit (increased/decreased) Sample from heparin lock or heparinized catheter
RI	PT: 9.5-13.5 s INR: 0.9 - 1.2 INR (warfarin): 2.0-3.0 INR (warfarin + cardiac condition OR LA): 2.5-3.5	APTT: 25-35s APTT (heparin): 1.5-2.5x mean)

Reflex/Follow-up Testing

Test (RI)	Info	Ordered When
Thrombin Time (15-20s)	Assesses deficiencies or dysfunction of fibrinogen (Factor I) or thrombin (Factor II) inhibitor Less fibrinogen available → longer clot time	Bleeding or thrombotic episodes Recurrent miscarriages BOTH PT and APTT unexpectedly prolonged	Congenital fibrinogen deficiency Heparin therapy Direct thrombin inhibitor therapy Contamination Presences of thrombin or fibrinogen inhibitors (rare) Acquired hypofibrinogenemia from liver disease or massive hemorrhage Abnormal fibrinolysis Increased products of clot breakdown interfering with fibrin polymerization (e.g., DIC)
Fibrinogen (2.2-5.0 g/L) (Critical: <1.0g/L with increased PT, APTT, TT)	Modified TT assay that estimates concentration of functional fibrinogen Clotting time correlates with active fibrinogen	Acquired hypofibrinogenemia Assess inflammatory conditions Recurrent miscarriages Suspected congenital fibrinogen deficiency or abnormality BOTH PT and APTT unexpectedly prolonged
D-Dimer	Measures fibrin clot breakdown products from plasmin Elevated D-dimer alone is non-specific except in VTE - Can be associated with a variety of diseases or inflammatory states Negative: normal, rules out DIC, PE, DVT Positive: may be associated with DIC, PE, or DVT		Deep vein thrombosis Pulmonary embolism Recent surgery/trauma Some cancers Acute/chronic infections or inflammatory diseases DIC Very heavy menstruation Normal pregnancy (high levels associated with complications) Healthy elderly patient Negative result can be used to exclude venousthromboembolism (VTE) in some populations
Mixing Studies
dRVVT	Assesses
Inhibitor Testing
Factor Assays

Abnormal Results

If PT/INR and APTT both prolonged

• Multiple factors in intrinsic and extrinsic pathways affected OR single factor deficiency in common pathway (I, II, V, or X [I if severe])
• Congenital:
  • Single factor deficiency for factor I, II, V, or X
• Acquired:
  • Multiple factor deficiency in intrinsic AND extrinsic pathways
    • Severe liver disease
    • Severe Vitamin K deficiency
    • DIC
  • Specific inhibitor directed to common pathway factor
  • Hypofibrinogenemia/dysfibrinogenemia
    • Massive hemmorage
    • Congenital
    • Fibrinolysis

If PT/INR is prolonged, but APTT is normal

• Probably related to factor VII
  • Congenital single factor VII deficiency
  • Acquired
    • Early liver disease
    • Early vitamin K deficiency
    • Early coumadin therapy or initial dose
    • Specific inhibitor to VII (rare)

If APTT is prolonged, but PT/INR is normal

• Probably related to intrinsic pathway factors VIII, IX, XI, or contact factors
  • Congenital
    • Single factor deficiency of VIII, IX, XI with bleeding (hemophilia A, B, or C respectively)
    • VWD (can lead to factor VIII deficiency)
    • Deficiency of factor XII, pre-K, HMWK (contact factors)
      • Prolongs APTT but doesn’t result in clinically significant bleeding
  • Acquired
    • Non-specific inhibitor (e.g., antiphospholipid antibodies)
      • Lupus anticoagulant in SLE patients
    • Specific inhibitor against intrinsic factors
      • Hemophilia patients may develop anti-FVIII or IX due to treatment
      • Acquired hemophilia or acquired VWD
    • Heparin therapy

Causes of Abnormal Results + Followup Testing

Test Affected	Cause	Disease States	Symptoms/Patient History	Pathways Affected	Followup Testing
BOTH PT and APTT	Single factor deficiency for I, II, V, or X				Factor assays
	Multiple factor deficiency in intrinsic AND extrinsic pathways	Severe liver disease	Jaundice, abdominal pain or tenderness		CBC (platelet count) RBC morphology (macrocytes and targets) Thrombin time/fibrinogen D-Dimer Factor V assay (decreased in liver disease but not Vit K deficiency) Factor VII assay (most sensitive factor) Mixing studies Liver enzyme panel Total and indirect bilirubin
		Severe Vitamin K deficiency			Factor V assay (decreased in liver disease but not Vit K deficiency)
		Disseminated intravascular coagulation			CBC (low platelets) RBC morph (appears as MAHA - schistocytes) Thrombin time (prolonged) Fibrinogen (low) D-Dimer (increased) Protein C, S, AT activity assays (<50%) Serum FDP (increased) Plasminogen, TPA, Plasminogen activator inhibitor-1 assay (decreased)
	Specific inhibitor to common pathway factor
	Hypofibrinogenemia/dysfibrinogenemia				Fibrinogen assay (reduced) Thrombin time (prolonged)
PT only	Factor VIII deficiency	Congenital single factor VII deficiency
		Early liver disease
		Early Vitamin K deficiency
		Early coumadin therapy/initial dose	Normal when on coumadin
		Specific inhibitor to VIII (rare)
APTT only		VWF disease	Young patient of any gender Mucocutaneous bleeding (nosebleeds, menorrhagia, etc.)	Factor VIII	CBC (PLT count to rule out thrombocytopenia) PT/APTT (to rule out other coagulation issues) VWF:Ag quantitative analysis immunoassay VWF:RCo Ristocetin co-factor qualitative analysis Factor VIII activity assay (FVIII:C) PAGE
		Hemophilia A, B, or C (single factor deficiency)	Male patient Anatomic soft tissue bleeding (deep bleeding in muscles, joints, organs) Wound oozing after trauma or surgery	Factor VIII (Hemophilia A - 85%) Factor IX (Hemophilia B - 14%) Factor XI or other factors (Hemophilia C - 1%)	Mixing studies (should be corrected) Factor assays (decrease in specific factor relevant to Hemophilia)
		Other congenital factor deficiencies			Factor assays
	Non-specific inhibitor	Lupus Anticoagulant/Antiphospholipid Syndrome (Autoimmune disease)	Female patient Recurrent miscarriages (3+ unexplained) Possibly mild hemolytic anemia (but may be normal) Thrombosis	Antibodies directed against phospholipids Promote coagulation in the body (but act as anticoagulant in vitro)	Mixing studies dRVVT (prolonged; corrected when test is performed with high phospholipid) Anti-nuclear antibody test or other immunoassays
		Heparin therapy	Normal when patient is on heparin. Can also be caused by specimen contamination with heparin.
		Other drugs	Rivaroxaban, apixaban, edoxaban
	Specific inhibitor against intrinsic factors		Usually against Factor VIII

Other disorders that may not affect PT/APTT include:

• Deficiencies
  • Antithrombin
  • Protein C
  • Free PS
  • TPA
• Mutations
  • Factor V (Factor V Leiden APCR)
  • Prothrombin (G20210A)
  • Fibrinogen (dysfribrinogenemia)
  • Plasminogen
  • PAI-1 (elevation)

Investigating Abnormal Results

1. Check specimen integrity and HCT
   1. Correct anticoagulant (sodium citrate)
   2. Proper tube filling (9:1 ratio, within 10% of fill volume)
   3. If HCT >55% (recollect sample using calculated dilution)
2. Check patient history for any conditions affecting coagulation. If present, report results:
   1. Anticoagulant therapy: warfarin/coumadin, heparin, direct oral anticoagulants (DOACs)
   2. Liver disease (early: PT, late: both PT/APTT)
   3. Vitamin K deficiency
   4. Known factor deficiency (single or multiple)
3. Look for other causes and perform reflex testing
   1. Decreased fibrinogen → Thrombin time or fibrinogen assay
   2. Specific single or multiple factor deficiency
   3. Inhibitor or antibody to factors
   4. Inhibitor or antibody interfering with test