Page 2631 - Hematology_ Basic Principles and Practice ( PDFDrive )

e12 Part XIII Consultative Hematology

Coagulation Screening Tests (PT, aPTT, Fibrinogen)
Summary points • PT, aPTT used to screen for coagulation factor deficiency and monitor anticoagulation; fibrinogen detects
hypofibrinogenemia, dysfibrinogenemia
• PT elevated with deficiencies of extrinsic and common pathway factors (PT expressed as INR when used to
monitor oral anticoagulant therapy)
• aPTT elevated with deficiencies of intrinsic and common pathway factors and inhibitors (heparin, lupus
anticoagulant, specific factor inhibitors)
Methodology Clot detection
Specimen requirements Citrate
Indications Suspected coagulation factor deficiency, monitor anticoagulation
Reference range Table 162.30
Interpretation • Reference ranges dependent on reagents and instrumentation, requiring laboratory-specific and population-
specific ascertainment (e.g., aPTT is normally prolonged during neonatal period)
• PT and aPTT vary in their sensitivity to deficiency of individual coagulation factors—PT tends to be least sensitive
to FII, aPTT to FIX
• Normal aPTT does not exclude mild but clinically significant factor deficiency (especially hemophilia B and von
Willebrand disease)
• Prolonged PT suggests deficiency of one or more factors (fibrinogen, II, V, VII, X) or presence of a coagulation
inhibitor
• Prolonged aPTT suggests deficiency of one or more factors (fibrinogen, II, V, VIII, IX, X, XI, or other contact
pathway factors) or presence of inhibitor such as lupus anticoagulant or heparin (consider heparin contamination
in specimen drawn from a heparinized line)
• Prolonged PT and aPTT suggests vitamin K deficiency, liver disease, disseminated intravascular coagulation,
common pathway factor deficiency
• Isolated prolonged PT (normal aPTT) suggests FVII deficiency
• Shortening of aPTT may reflect elevated FVIII level (caused by inflammation)
• Spurious prolongation of tests with polycythemia, partially filled collection tube, prolonged storage or warming,
clotted specimen
Related tests Coagulation factor levels (fibrinogen, factors II, V, VII, VIII, IX, X, XI, other contact pathway factors), lupus
anticoagulant, mixing studies
aPTT, Activated partial thromboplastin time; FII, factor II; FIX, factor IX; FVII, factor VII; FVIII, factor VIII; INR, international normalized ratio; PT, prothrombin time.

Coagulation Factor Levels (Fibrinogen, II, V, VII, VIII, von Willebrand Factor, IX, X, XI, Contact Pathway Factors)
Summary points • Choice of which specific factor(s) to measure driven by clinical scenario and screening test results (e.g., isolated
prolonged PT suggests FVII deficiency, isolated preoperative aPTT suggests measurement of at least FVIII, FIX,
and FXI, to assess bleeding risk)
• Low von Willebrand factor may not result in abnormal screening test results
Methodology Clot detection
Specimen requirements Citrate
Indications Suspected coagulation factor deficiency, especially as suggested by prolonged coagulation screening test and
subsequent mixing study that corrects, assessment of risk factors for thrombosis (e.g., high FVIII), monitoring of
factor-replacement therapy in patients with coagulopathy
Reference range Tables 162.30 and 162.32
Interpretation • No definitive threshold for what constitutes hemostatically adequate factor level, but criteria for diagnosing mild
factor deficiency often set at 40%–50%
• Fibrinogen, FVIII, and von Willebrand factor are acute-phase reactants
• Factor deficiency may be due to reduced production (hereditary factor deficiency, liver disease, vitamin K
deficiency), increased consumption (disseminated intravascular coagulation), or specific inhibitor (FVIII)
• Acquired deficiency of multiple factors occurs with liver disease, vitamin K deficiency (including vitamin
K–antagonist therapy or toxicity), disseminated intravascular coagulation—FV, FVII, and FVIII may aid in
distinguishing between these
• Rare inherited combined factor deficiencies exist (combined FV/FVIII, vitamin K–dependent factors)
• With exception of FXI, contact factor deficiencies (FXII, high-molecular-weight kininogen, prekallikrein) may cause
prolonged aPTT but are not associated with increased bleeding risk
• Vitamin K–dependent and contact factors are physiologically low in early infancy
• FV and FVIII are particularly labile, with potential for spuriously low levels—carefully consider abnormally low
levels in clinical and laboratory context and assess need for repeat/confirmatory testing
Related tests Specific factor inhibitor testing, lupus anticoagulant, mixing studies
aPTT, Activated partial thromboplastin time; FIX, factor IX; FV, factor V; FVII, factor VII; FVIII, factor VIII; FXI, factor XI; FXII, factor XII; PT, prothrombin time.