Limitations
of aPTT
Inflammation:
Hyperfibrinogenemia
(>400 mg/dL)
Conditions that cause aPTT to be insensitive to heparin
“heparin resistance”:
Causes a shortening of
aPTT making it less sensitive to heparin changes
Factor VIII (150%)
Depleted antithrombin
PF4 neutralizes
heparin
Release of PF4 from platelets in vitro: if platelets not seperated from plasma
aPTT stays in
therapeutic range even with increased heparin dosage
Dysfibrinogenemia, FDP,
paraproteins:
Cause a prolonged aPTT
which may be interpreted as too much heparin