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Activated Partial Thromboplastin Time (aPTT) 1301

due to inflammatory disease without bleeding is subject to observer variability. Difficult Pearls
tendency. venipuncture causing exposure to tissue factor • Insensitive test that may be useful cage side
VetBooks.ir Next Diagnostic Steps to Consider Specimen Collection and Handling • Carriers of hemophilia (coagulation factors
for suspected warfarin-intoxicated dogs if
and platelet activation may falsely shorten ACT.
aPTT unavailable.
if Levels Are High
Confirm with more sensitive tests (e.g., aPTT,
hereditary factor VII deficiency will not be
prewarmed to 37°C. Mix by inversion, incubate
platelet count). Whole blood into siliceous (gray top) tube, are 40%-60% of normal) and beagles with
at 37°C (purpose made heating block, water detected.
Drug Effects bath, or human axilla) and check for clot forma-
Prolonged by anticoagulant therapy (e.g., tion every 5-10 seconds. Do not incubate in a AUTHOR & EDITOR: Lois Roth-Johnson, DVM, PhD,
DACVP
heparin), some antibiotics (newer lipoglyco- closed palm, as it does not provide a consistent
peptide antibiotics, rarely used in veterinary 37°C environment.
medicine, may interfere with factor Xa).
Relative Cost: $
Lab Artifacts
An in-office test. Any variation in protocol may
alter results. Endpoint (visual clot detection)

Activated Partial Thromboplastin Time (aPTT)

Definition antagonism, factor dilution (massive • Prolonged:
Screening test for disorders of intrinsic and transfusion/fluid replacement) ○ Inappropriate ratio of plasma to citrate:
common coagulation pathways. Named for • Severe von Willebrand disease: von Wil- underfilling of citrate tube, polycythemia
additives (e.g., partial thromboplastins, surface lebrand factor stabilizes factor VIII, so severe ○ Sample collection: Difficult/traumatic
activators) used to activate coagulation in vitro. deficiency (<30%) can lead to increased venipuncture, improper storage
During testing calcium is added to overcome aPTT. ○ Anti-phospholipid antibodies: rarely
the effect of citrate, and aPTT is the time from • Anticoagulant therapy (e.g., heparin, warfarin) documented in animals, but may inter-
addition of calcium chloride to clot detection. fere with phospholipids in testing phase
Next Diagnostic Steps to Consider even though animals don’t have signs of
Synonym(s) if Levels Are High hypocoagulation
aPTT, APTT, PTT Interpret with signalment, history (e.g., drug/
toxin exposure) and examination findings, Specimen Collection and Handling
Physiology other laboratory findings, and other markers Citrated whole blood (blue top tube) collected
There is extensive cross talk between the intrin- of hemostasis (e.g., platelets, PT, fibrinogen) via atraumatic phlebotomy. Fill to the appropri- Laboratory Tests
sic and extrinsic coagulation cascades in vivo. (p. 433). ate level (markings may vary by manufacturer).
Separation of the two in laboratory testing (e.g., Centrifuge and separate plasma within 1 hour,
PT, aPTT) is artifactual but useful to narrow Next Diagnostic Steps to Consider store/ship refrigerated in plastic tube (additive
differentials for coagulopathy. aPTT screens for if Levels Are Low free). If testing > 4 hours from collection, freeze
deficiencies in intrinsic/contact (PK, XII, XI, Shortened aPTT cannot be used to confirm for storage/shipment. Contact laboratory before
IX, VIII) and common (X, V, II, fibrinogen) hypercoagulability. collection as sample handling policy may vary.
pathway factors and is prolonged when activity
of a single factor is decreased by 70% or with Important Interspecies Differences Relative Cost: $-$$
multiple factor deficiencies. Cats: factor XII deficiency causes prolonga-
tion in aPTT with normal PT and normal Pearls
Reference Interval hemostasis. Prolonged aPTT from unknown • Low aPTT values do not indicate a proco-
Laboratory, instrument, and species specific causes in cats with effective hemostasis has also agulant state.
reference intervals must be used. Approximate been described. • Unfractionated heparin will prolong aPTT
intervals, dogs: 9-17 seconds; cats: 15-19 but not PT.
seconds Drug Effects • Prolonged aPTT may be seen with a normal
Increased: anticoagulant therapy (e.g., heparin, PT in the early stages of DIC.
Causes of Abnormally High Levels warfarin)
• Single factor deficiency: any intrinsic/contact AUTHOR: Mary Leissinger, DVM, MS, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
or common pathway factor (e.g., hemophilia Lab Artifacts
A [VIII], hemophilia B [IX] in dogs, and • Shortened or normalized
XII deficiency in cats) ○ Inappropriate ratio of plasma to citrate:
• Multiple factor deficiencies: disseminated overfilling citrate tube, severe anemia
intravascular coagulation (DIC), liver disease ○ Sample collection: difficult/traumatic
(cholestatic, failure), vitamin K deficiency/ venipuncture

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