Prothrombin time (PT) and activated partial thromboplastin time (aPTT) are paired citrate-plasma screening tests for deficiencies of selected clotting factors. They look at secondary hemostasis, the part of clotting that turns fibrinogen into a fibrin clot. A coagulation profile is not a universal test of bleeding safety. It does not count platelets, test platelet function, or measure von Willebrand factor. It is insensitive to excess factor activity, so it is not a validated test for a clot. It never supplies a diagnosis by itself, and it never supplies a drug dose.

When evaluating a bleeding dog or preparing a patient for invasive surgery, clinicians frequently order PT and aPTT as a routine safety screen. Interpreting the results requires understanding exactly which coagulation pathways are being challenged in the test tube, recognizing the substantial clinical disorders that leave these tests completely normal, and enforcing strict pre-analytical sample handling rules. Without proper collection protocols and laboratory-specific reference intervals, seconds on a printout can easily mislead clinical decisions.

What PT and aPTT screen for in a dog

Hemostasis in dogs is broadly divided into primary hemostasis, the platelet plug at a vessel injury, and secondary hemostasis, the plasma-factor sequence that cleaves fibrinogen into fibrin. Routine canine PT and aPTT assays evaluate secondary hemostasis in platelet-poor plasma, outside the vessel.

Both tests are performed on citrated plasma. Sodium citrate binds ionic calcium in the whole-blood collection tube, halting the coagulation cascade at the moment of venipuncture. In the testing laboratory or point-of-care analyzer, calcium is added back in the presence of specific biochemical activators, and an automated optical or mechanical sensor measures the exact elapsed time in seconds until detectable fibrin strands polymerize:

• Prothrombin Time (PT): Assays the extrinsic pathway and the common pathway. The plasma is incubated with tissue factor (thromboplastin) and calcium. That activates factor VII, then the common pathway: factor X, factor V, prothrombin (factor II), and fibrinogen. Factor VII is the only factor unique to the extrinsic limb, so an isolated PT prolongation points toward factor VII, early vitamin K antagonism, or early hepatic synthetic failure. It does not prove which of those is present.

• Activated Partial Thromboplastin Time (aPTT): Assays the intrinsic pathway and the common pathway. The plasma is incubated with a contact activator, such as ellagic acid, kaolin, or silica, plus phospholipid and calcium. The 2025 JAVMA review describes the contact group as prekallikrein, high-molecular-weight kininogen, and factor XII, which then activate factors XI, IX, and VIII and the common pathway (factors X, V, II, and fibrinogen). Because the reagent supplies phospholipid, the assay does not use the patient's platelets.

flowchart TD
    subgraph Primary ["Primary Hemostasis (Not Measured by PT/aPTT)"]
        Plt["Platelets (Number & Function)"]
        vWF["von Willebrand Factor (vWF)"]
        Endo["Endothelial Adhesion"]
    end
    subgraph Screen ["Citrated Plasma Secondary Hemostasis Screen"]
        subgraph Intrinsic ["Intrinsic Pathway (aPTT)"]
            F12["Factor XII (Contact)"] --> F11["Factor XI"]
            F11 --> F9["Factor IX"]
            F9 --> F8["Factor VIII:C"]
        end
        subgraph Extrinsic ["Extrinsic Pathway (PT)"]
            F7["Factor VII + Tissue Factor"]
        end
        subgraph Common ["Common Pathway (Both PT & aPTT)"]
            F10["Factor X + Factor V"] --> F2["Prothrombin (Factor II)"]
            F2 --> F1["Fibrinogen (Factor I)"]
            F1 --> Clot["Insoluble Fibrin Clot"]
        end
    end
    F8 --> F10
    F7 --> F10

PT and aPTT are built to detect factor deficiency by a longer time. A prolonged result can mean that one or more factors in the triggered pathway are low, or that an inhibitor such as heparin is interfering. The 2025 JAVMA review states that the assays are insensitive to excessive factor activity. They also do not detect bleeding from vessel fragility or from a platelet problem.

Four patterns, and the bleeding problems each one still leaves open

When interpreting canine coagulation results, clinicians evaluate the combination of PT and aPTT times side by side. The biological behavior of each pathway creates four distinct diagnostic patterns. While each pattern narrows the anatomical location of a potential defect, none of them provides an immediate standalone diagnosis. Each pattern leaves specific clinical questions unresolved that require secondary testing, patient history, and physical examination to clarify.

Pattern 1: Normal PT and normal aPTT (the silent bleeding risks)

A completely normal coagulation screen in a dog presenting with petechiae, ecchymoses, spontaneous epistaxis, or postoperative oozing is a classic clinical paradox. It occurs because PT and aPTT completely bypass primary hemostasis. Platelet adhesion, platelet activation, and platelet aggregation on exposed subendothelial collagen happen before the fluid-phase clotting cascade is required.

The most common inherited bleeding disorder in domestic dogs is von Willebrand disease (vWD). Von Willebrand factor tethers platelets to a damaged vessel wall and also circulates with factor VIII. PT and aPTT reagents supply phospholipid, so the tube does not need the dog's von Willebrand factor. The 2025 JAVMA review calls canine von Willebrand disease the most common inherited bleeding disorder in dogs and states that coagulation screening tests are typically normal, so a specific von Willebrand factor assay is required. The Merck platelet chapter says to suspect it when platelet-type bleeding, a normal platelet count, normal PT and aPTT, and a long buccal mucosal bleeding time occur together, and to confirm it with a low plasma von Willebrand factor concentration or a relevant DNA result. Type 1 disease is the most common form and is often mild to moderate; many affected dogs are subclinical. Occasionally an affected dog has low factor VIII activity and then a prolonged aPTT. A normal pair is usual. It is not proof the dog is safe for surgery, and a prolonged aPTT is not how this disease is ruled out.

Similarly, severe primary hemostatic failure resulting from immune-mediated thrombocytopenia (ITP), other causes of a low platelet count, or a platelet-function disorder can leave PT and aPTT unaltered. Standard reagents can also miss mild or moderate factor loss, and the miss depends on the reagent. Mischke and Nolte (1997) found that standard PT methods detected 24% to 58% of 50 abnormal canine plasma samples with extrinsic-pathway factor loss, while an optimized dilution method reached 90% to 100%. Mischke (2003) found that, across standard PT reagents, factor VII activity could be as low as 16% to 39% of a pooled normal before PT reached the upper reference limit. Mischke (2000) tested five commercial aPTT reagents, used as the manufacturers instructed, on 30 canine plasmas with minor or moderate intrinsic-pathway deficiencies and found sensitivities from 67% to 100%. One reagent detected all 30, so aPTT does not always miss mild disease. These studies used prepared or selected plasmas. They are not the sensitivity of an unnamed analyzer in current clinics, and they do not predict surgical bleeding. A single cutoff such as 20% to 40% would pretend the reagents agree.

Pattern 2: Prolonged PT with normal aPTT (extrinsic pathway localization)

When prothrombin time is prolonged while aPTT remains within the laboratory reference interval, the defect is localized specifically to the extrinsic pathway. Because the common pathway (factors X, V, II, and fibrinogen) is shared by both tests, a defect in any common-pathway protein would prolong aPTT as well. Therefore, an isolated PT prolongation directs diagnostic focus squarely onto factor VII.

Clinically, isolated PT prolongation is most frequently observed in the early phase of anticoagulant rodenticide poisoning. Anticoagulant rodenticides block vitamin K recycling. Merck lists first-generation examples such as diphacinone and chlorophacinone and second-generation examples such as brodifacoum and bromadiolone. The liver still produces factors II, VII, IX, and X, but they are not carboxylated into the functional form. Cornell's coagulation-assay teaching page says factor VII has the shortest half-life of these factors, about 6 hours in the dog, so PT may prolong before aPTT, including within about 24 hours, often before the dog is bleeding. The same page says hemorrhage usually appears when prothrombin is also deficient, by which time aPTT should be prolonged, and that this can take about 48 hours in dogs. Those intervals are a teaching sequence, not a clock that every dog and every reagent will follow, and they are not a vitamin K dose. Which bait was eaten, and how it is treated, is the subject of the rodenticide article.

Isolated PT prolongation can also reflect congenital factor VII deficiency. Merck reports it in Beagles, English Bulldogs, Alaskan Malamutes, Alaskan Klee Kais, Miniature Schnauzers, Boxers, and mixed-breed dogs. Inheritance is an autosomal pattern with incomplete dominance, not a simple recessive trait. It is usually not associated with spontaneous bleeding. Affected dogs can bruise or bleed longer after surgery, and the deficiency is often found on a preoperative screen. Vitamin K deficiency from fat malabsorption is a separate acquired cause: Merck cites too little bile, as in biliary obstruction, lymphangiectasia, or severe villous atrophy. Early hepatic synthetic failure can also prolong PT before aPTT moves. None of these is diagnosed by the PT pattern alone.

Pattern 3: Normal PT with prolonged aPTT (intrinsic pathway and contact anomalies)

An isolated aPTT prolongation with a normal PT localizes the defect to contact or intrinsic factors: factor XII, factor XI, factor IX, or factor VIII, and the contact proteins grouped with them. The same pattern can be a contact-factor deficiency with no clinical bleeding or a hemophilia that does cause bleeding. The seconds do not say which.

At one end of the spectrum is contact-factor deficiency—most notably factor XII (Hageman factor) deficiency. In the tube, aPTT depends on contact activation. When factor XII is deficient, that in vitro start is delayed and aPTT prolongs on the laboratory's own scale. In the animal, hemostasis can start through tissue factor and factor VII, so factor XII is not required for clinical clotting. Canine factor XII deficiency, reported in German Shorthaired Pointers, Standard Poodles, and a Miniature Poodle family, prolongs aPTT without clinical bleeding. Factor XII deficiency is described as common in cats. That prevalence is not a canine figure; in dogs the published reports are the breeds named here. A dog with this deficiency has no clinical bleeding diathesis. Treating that result as hemophilia, or canceling a needed procedure on the number alone, treats a contact delay as a bleeding disorder. A specific factor assay is what separates the two.

At the opposite end of the spectrum are the inherited sex-linked recessive hemophilias. Hemophilia A (factor VIII coagulant deficiency) and Hemophilia B (factor IX deficiency) typically affect males. Affected females are rare and are described in closely inbred families. Signs can include umbilical or gingival bleeding in puppies, hemarthrosis, hematomas, and bleeding into a body cavity or after surgery. For hemophilia A, Merck says dogs with factor VIII under about 5% of normal often bleed spontaneously, while dogs with about 5% to 10% more often bleed after trauma or surgery. Affected animals usually have factor VIII under 10% and a prolonged aPTT. Carriers of hemophilia A or B often have about 40% to 60% activity, and their PT and aPTT are usually normal, so a normal screen does not identify a carrier. This page does not give a transfusion dose.

Factor XI deficiency has been reported in Kerry Blue Terriers, one English Springer Spaniel, a Great Pyrenees, and Weimaraners. When activity is about 30% to 40% of normal or lower, bleeding after trauma or surgery can be mild and is often delayed by a few days. In a dog younger than about 6 months, Merck says an immature liver can lower factor production, so a borderline screen needs caution and may not match the adult picture. Unfractionated heparin prolongs aPTT and is not expected to prolong PT; low-molecular-weight heparin does not affect these screens the same way. A heparinized catheter is a sample problem, not a diagnosis: Cornell says to flush an indwelling catheter with saline, not heparin, and to discard an aliquot before drawing the citrate sample. JAVMA describes a discard of about 0.5 to 1.5 mL, and Cornell's sampling page describes about 3 to 5 mL. Follow the laboratory that will receive the tube.

Pattern 4: Both PT and aPTT prolonged (common pathway, consumptive, or hepatic failure)

When both PT and aPTT are prolonged, the dog may have a common-pathway deficiency (factor X, V, II, or fibrinogen) or more than one factor deficiency. The pattern needs a structured workup. It does not name rodenticide toxicity, liver failure, or DIC.

Situations that can produce dual prolongation include:

• Advanced Anticoagulant Rodenticide Toxicosis: After anticoagulant bait ingestion, loss of factor VII can be followed by loss of factors IX, X, and II, and both PT and aPTT can then prolong. How many seconds that is depends on the assay. Bait class and antidote decisions stay with the rodenticide article.

• Severe Hepatic Dysfunction: Most coagulation proteins are made in the liver. von Willebrand factor is made by endothelium and megakaryocytes, and part of factor VIII is extrahepatic, so those proteins do not fall the same way. Merck's coagulation-protein chapter says aPTT, PT, or both are prolonged in a textbook estimate of 50% to 75% of dogs with severe liver disease, and that bleeding tendency is still hard to predict. The hepatic-testing chapter adds that conventional PT and aPTT may not reflect rebalanced hemostasis, because anticoagulant proteins such as antithrombin and protein C can fall along with the procoagulants. A human observation that biopsy bleeding rose above 1.5 times the upper limit is not a canine transfusion or biopsy cutoff.

• Disseminated Intravascular Coagulation (DIC): DIC is not a primary disease. Merck describes it as a syndrome secondary to severe infection, heat stroke, burns, neoplasia, or severe trauma, often through sepsis or a systemic inflammatory response. Early DIC can be hypercoagulable. Later, consumption of coagulation proteins and platelets can produce a hypocoagulable state and overt bleeding. Diagnosis is not standardized. It is often based on at least three abnormal hemostasis findings plus a predisposing disease, not on PT and aPTT alone.

• Severe Hypofibrinogenemia: Both assays wait for fibrin, so a very low fibrinogen can prolong PT and aPTT even without a separate factor deficiency. Cornell's teaching page says fibrinogen below 75 mg/dL in the dog can do that, and that sensitivity to low fibrinogen at Cornell runs from thrombin time, to aPTT, to PT. That 75 mg/dL figure is a teaching threshold for prolongation. It is not Cornell's fibrinogen reference interval, which is 150 to 490 mg/dL by the Clauss method, and it is not a universal cutoff for every analyzer.

A normal or abnormal time is not the same as a bleeding risk

A common clinical assumption is that a patient's coagulation time correlates directly and linearly with their clinical bleeding risk—that a normal time guarantees safety, and a prolonged time predicts inevitable hemorrhage. Modern clinical and veterinary evidence demonstrates that this assumption is fundamentally flawed.

In a retrospective hospital series, Yagneswar et al. (2018) studied 203 dogs with a clinical reason for coagulation testing on one turbidimetric analyzer, the ACL-TOP CTS 300. Dogs with known anticoagulant rodenticide exposure and dogs given blood products in the prior 28 days were excluded, so the series does not describe those situations. Using adapted human trial criteria, 28 of 203 dogs (14%) had clinically significant bleeding. Neither PT nor aPTT was statistically associated with that bleeding. The proportion of dogs that bled was similar whether the time was below, inside, or above the interval: PT was low in 1 of 7 dogs (14%), inside the interval in 16 of 128 (12%), and high in 11 of 68 (16%); aPTT was low in 2 of 10 (20%), inside in 11 of 109 (10%), and high in 15 of 84 (18%). Of 38 dogs with both times prolonged, 7 (18%) met the bleeding definition. The authors note other canine studies in which prolongation was likewise a poor separator of clinical bleeding, including some DIC cases. That does not mean a long time is irrelevant in hemophilia, anticoagulant rodenticide, or acute hemorrhage. It means these seconds, on this analyzer and in this population, did not sort bleeders from non-bleeders.

Conversely, questions frequently arise regarding the diagnostic meaning of a shortened coagulation time—a PT or aPTT that clots faster than the lower limit of the reference interval. Because PT and aPTT were engineered specifically as deficiency screens triggered by massive excess concentrations of artificial reagents, standard teaching (such as that provided by Cornell eClinPath) emphasizes that only prolongation is physiologically validated. However, a retrospective teaching hospital series by Song, Drobatz, and Silverstein (2016) compared 23 dogs with a shortened PT or aPTT with 23 dogs whose times were normal. The shortened group had more catheter or vessel thrombus, more suspected pulmonary thromboembolism, and higher D-dimer. The authors concluded that a shortened result may indicate hypercoagulability and asked for a prospective study. The published title says 25 cases; the comparison in the abstract is 23 dogs in each group. The dogs already had thromboelastography, so they were not a general screening population.

Clinicians must avoid upgrading the Song et al. association into an absolute rule: a shortened PT or aPTT does not prove active thrombosis or justify starting aggressive antithrombotic or anticoagulant therapy without corroborating clinical evidence. Simultaneously, the study reminds clinicians not to dismiss shortened values entirely as meaningless noise. Both prolongation and shortening must be interpreted within the dog's complete hemodynamic, inflammatory, and anatomical context.

Sample problems that look like a factor deficiency

Coagulation results are easy to distort before they reach the analyzer. A difficult draw, the wrong tube, the wrong fill, a clot, a delay, cold whole blood, or frozen whole blood can imitate a factor deficiency or hide one. The seconds are only as good as the collection record.

To ensure diagnostic validity, clinical teams must adhere to strict sample handling principles:

• The Only Acceptable Anticoagulant: Buffered Sodium Citrate (Light Blue Top). Use 3.2% or 3.8% sodium citrate, the light-blue-top tube. Cornell and the 2025 JAVMA review both name those concentrations. EDTA, heparin, serum, serum-separator, and clot-activator samples are invalid and should be redrawn. On a multi-tube draw, JAVMA says to collect citrate before other additive tubes so EDTA or heparin does not contaminate it, or to collect a plain no-additive tube first if serum is also needed. Do not draw into a dry syringe and transfer the blood into citrate afterward. JAVMA and Cornell both warn that a clot can start in the syringe before the anticoagulant is mixed.

• Strict 1:9 Citrate-to-Blood Fill Ratio. The ratio is 1 part citrate to 9 parts whole blood. Underfilling leaves extra citrate, which can bind the calcium the analyzer adds and prolong both times. Overfilling leaves too little citrate, so a clot can consume factors before testing. Cornell's syringe examples are 0.2 mL citrate plus 1.8 mL blood, 0.3 mL plus 2.7 mL, and 0.4 mL plus 3.6 mL. If the vacuum tube or syringe is short of the intended volume, redraw it. The choice between 3.2% and 3.8% is a smaller question than underfilling, and it is covered below.

• Venipuncture Technique: Clean, Atraumatic Draws. A traumatic draw can activate clotting in the needle. Cornell's teaching page says aPTT is more sensitive than PT to that artifact and may be prolonged, falsely shortened, or falsely normalized. Inspect the sample. JAVMA and Cornell both say clot fragments mean the draw failed and the sample should be replaced. Hemolyzed plasma should also be redrawn unless the dog has in vivo hemolysis.

• Rapid Centrifugation to Platelet-Poor Plasma (PPP). Hold citrated whole blood at room temperature until it is spun. JAVMA says not to refrigerate it first, because cold can precipitate or activate factors, and recommends centrifugation within about 1 hour. For most routine assays the review cites a spin at or above 1,500 × g for 10 to 15 minutes. Cornell's instructions are stricter: greater than 2,500 × g for 10 to 15 minutes. Use the receiving laboratory's speed. One hospital series called plasma platelet-poor when the count was under 10,000 per microliter after a 2,800 × g spin. That was that laboratory's target, not a number to copy onto every chart.

• Temperature Handling: Never Freeze Whole Blood. Do not refrigerate whole blood before centrifugation, and do not heat or freeze whole blood. JAVMA lists prolonged delay, heating, and freezing of whole blood as reasons to reject a sample. Separate the plasma, then refrigerate it for a same-day shipment or freeze it. Cornell allows frozen plasma to be stored for up to 2 weeks. JAVMA says frozen storage is preferable when testing or shipping will be delayed, because it protects labile factors such as factor VIII, and that overnight delivery on frozen cold packs is enough for most clinical tests. Dry ice is for certain specialized assays. Ask the receiving laboratory rather than treating dry ice, or a specific freezer temperature, as the routine rule.

• Severe Polycythemia (Hematocrit >65%). Citrate stays in the plasma. JAVMA says a hematocrit above 65% may over-citrate that smaller plasma volume and gives this adjustment: citrate volume in milliliters = 0.00185 × (100 − hematocrit percent) × blood volume in milliliters. The review takes the formula from the human CLSI H21 guideline, 6th edition, 2024. It is a cited laboratory adjustment, not a canine outcome trial, and it is not a calculation that diagnoses a bleeding disorder.

A common clinical query involves the choice between 3.2% and 3.8% buffered sodium citrate. In a rigorous investigation by Stokol et al. (2000) compared 3.2% and 3.8% citrate in 30 healthy dogs and 12 dogs with hereditary hemostatic disorders. Median PT, fibrinogen, factor VIII, and factor IX did not change significantly on the systems tested. aPTT was mildly prolonged with 3.2% citrate on one mechanical system, and that effect depended on the animal, the instrument, and the reagent. Either concentration can be acceptable. The laboratory still has to know which tube was used, and the reference interval has to belong to that method.

Why the number of seconds belongs to one laboratory

One of the most persistent misunderstandings among pet owners and medical teams is treating clotting times in seconds as universal physical constants, comparable to body temperature or blood glucose. In reality, a clotting time in seconds is entirely an artifact of the specific instrument, the reagent manufacturer, the tissue source of the thromboplastin, the contact activator chemistry, and the mechanical or optical detection threshold used to register clot formation.

This distinction becomes starkly apparent when comparing reference intervals between central reference laboratories and in-clinic point-of-care (POC) analyzers. Consider the published canine reference intervals from the Cornell University Comparative Coagulation Laboratory alongside the benchmark findings of Tseng, Hughes, and Giger (2001), who evaluated an in-clinic point-of-care coagulation analyzer across 27 healthy and 32 diseased dogs:

The two rows are not interchangeable normals. On Cornell's published ellagic-acid plasma method, an aPTT of 18 seconds is above the listed upper limit of 17.5 seconds. On the 2001 whole-blood device, an aPTT of 80 seconds sits inside 72.5 to 100.3 seconds. Reading the 80-second result against Cornell's interval would invent a severe factor deficiency the device did not report. Cornell's public table does not state how many dogs were used or when the interval was built, and the cat, horse, and other species rows are different.

Tseng, Hughes, and Giger studied that one device in 27 healthy and 32 diseased dogs. On citrated whole blood, sensitivity and specificity against the laboratory plasma method were 85.7% and 95.5% for PT, and 100% and 82.9% for aPTT. Agreement was 93% for PT and 87.5% for aPTT, and it was still incomplete. With noncitrated whole blood, PT agreement fell to 73%. Good agreement on one 2001 device is not a specification for an analyzer sold now. A number of seconds without that method's reference interval cannot be interpreted.

What to put on the record before anyone interprets the result

Because pre-analytical handling errors and clinical history dictate how a coagulation time must be interpreted, test results should never stand alone on a clinical handover sheet or electronic medical record. As emphasized by McKown, Brooks, and Goggs (JAVMA 2025), optimal diagnostic outcomes require rigorous collaboration between clinicians and laboratory diagnosticians.

Before interpreting a coagulation panel—or transferring an emergency patient to a referral ICU—the clinical team should record the following six essential parameters directly on the laboratory log:

1. Patient Demographics and Signalment: Record species, breed, and age. Breed can point to a reported susceptibility: Doberman Pinschers are among the breeds in which von Willebrand disease is relatively common; German Shorthaired Pointers and Standard Poodles are among the breeds reported with factor XII deficiency that does not cause bleeding; Beagles are among the breeds reported with factor VII deficiency. Age matters because a dog younger than about 6 months can have lower factor production from an immature liver, so the screen may not match the adult picture.

2. Clinical Hemostatic Presentation: Document whether the patient exhibits active bleeding, and characterize the bleeding phenotype. Petechiae, mucosal hemorrhages, and scleral ecchymoses point toward primary hemostatic failure (platelets or vWF). In contrast, deep hematomas, hemarthrosis, or intracavity bleeding (hemoabdomen, hemothorax) suggest secondary coagulation factor failure. If the test is an asymptomatic preoperative screen (e.g., prior to rhinoscopy or organ biopsy as discussed in canine nasal discharge workups or hematuria evaluations), state this clearly.

3. Pre-Analytical Sampling Details: Log the collection date, exact collection time, anatomical venipuncture site, needle gauge, and tube type (3.2% vs. 3.8% buffered sodium citrate). Verify and note that the tube was filled to the precise 1:9 ratio mark, that the sample was confirmed free of gross fibrin clots, and record the visual grade of any hemolysis or lipemia.

4. Timing of Prior Medical Interventions: Explicitly document whether the blood sample was collected before or after the administration of intravenous crystalloid or colloid fluids (which induce dilutional coagulopathy), blood products (fresh frozen plasma, cryoprecipitate, or whole blood, which instantly replenish circulating factors), heparin flushes, or antifibrinolytic drugs (tranexamic acid or aminocaproic acid). Post-transfusion coagulation testing measures donor factors, obscuring the patient's underlying baseline disease.

5. Concurrent Platelet Count and Hematocrit: Coagulation testing must never occur in an isolated vacuum. Always record the concurrent automated platelet count and manual blood smear review from the patient's complete blood count. Confirming whether thrombocytopenia is present immediately differentiates solitary coagulopathies from dual defects (such as DIC or massive hemorrhage). Additionally, record the packed cell volume (hematocrit) to verify whether severe polycythemia (>65%) required citrate volume adjustment.

6. Testing Facility, Methodology, and Specific Reference Intervals: State whether the test was performed on an in-clinic point-of-care analyzer or processed by an accredited reference laboratory. Transcribe the testing facility's exact method-matched reference intervals in seconds alongside the patient's measured values, rather than writing only the raw numbers.

When a coagulation screen remains ambiguous or fails to align with the patient's clinical bleeding severity, veterinary teams should branch into targeted secondary panels rather than repeating basic screens. As reflected in the Cornell Comparative Coagulation submission menu, targeted panels provide the required resolution:

• Hemostasis Panel: Cornell's hemostasis panel is the coagulation panel, meaning aPTT, PT, and thrombin time, plus von Willebrand factor antigen. It does not replace a platelet count or a platelet-function test.

• Hemophilia Panel: Cornell's hemophilia panel adds factor VIII and factor IX activity on top of the hemostasis panel. Those assays are what separate hemophilia A, hemophilia B, and a contact-factor delay.

• Consumptive / DIC Panel: Cornell's DIC panel is aPTT, PT, fibrinogen, antithrombin, and D-dimer. It adds the tests a PT and aPTT pair leaves out. Submitting it does not by itself diagnose DIC.

Keep the pair in its lane. Report the pattern, the laboratory's method and interval, the platelet count, and the collection facts. A normal pair does not clear a dog for surgery, biopsy, or a dental extraction. A prolonged pair does not name the disease. No result on this page is a drug dose.

Sources

1. McKown, M., Brooks, M. B., & Goggs, R. (2025). Collaboration between clinicians and laboratory diagnosticians can improve outcomes for patients with coagulation disorders. Journal of the American Veterinary Medical Association, 263(S1), javma.24.11.0746.

2. Cornell University College of Veterinary Medicine, Animal Health Diagnostic Center. Comparative Coagulation sampling instructions.

3. Cornell University College of Veterinary Medicine, Animal Health Diagnostic Center. Comparative Coagulation assay reference intervals.

4. Cornell University College of Veterinary Medicine, Animal Health Diagnostic Center. Comparative Coagulation submission guidelines.

5. Mischke, R., & Nolte, I. (1997). Optimization of prothrombin time measurements in canine plasma. American Journal of Veterinary Research, 58(3), 236–241.

6. Mischke, R. (2000). Activated partial thromboplastin time as a screening test of minor or moderate coagulation factor deficiencies for canine plasma: sensitivity of different commercial reagents. Journal of Veterinary Diagnostic Investigation, 12(5), 414–419.

7. Mischke, R. (2003). Sensitivity of different prothrombin time assays to factor VII deficiency in canine plasma. The Veterinary Journal, 166(2), 127–130.

8. Merck Veterinary Manual. Coagulation Protein Disorders in Animals.

9. Merck Veterinary Manual. Platelet Disorders in Animals.

10. Merck Veterinary Manual. Coagulation Tests in Hepatic Disease in Small Animals.

11. Yagneswar, P. K., et al. (2018). Retrospective comparison of standard clotting tests and novel clot waveform parameters in dogs using the turbidimetric ACL-TOP CTS 300 coagulation analyzer. Journal of Veterinary Diagnostic Investigation, 30(6), 843–851.

12. Tseng, L. W., Hughes, D., & Giger, U. (2001). Evaluation of a point-of-care coagulation analyzer for measurement of prothrombin time, activated partial thromboplastin time, and activated clotting time in dogs. American Journal of Veterinary Research, 62(9), 1455–1460.

13. Stokol, T., Erb, H. N., & Brooks, M. B. (2000). Effect of citrate concentration on coagulation test results in dogs. Journal of the American Veterinary Medical Association, 217(11), 1672–1677.

14. Song, J., Drobatz, K. J., & Silverstein, D. C. (2016). Retrospective evaluation of shortened prothrombin time or activated partial thromboplastin time for the diagnosis of hypercoagulability in dogs: 25 cases (2006-2011). Journal of Veterinary Emergency and Critical Care, 26(3), 398–405.

15. eClinPath, Cornell University College of Veterinary Medicine. Coagulation assays.