Why are my APTT (Activated Partial Thromboplastin Time), INR (International Normalized Ratio), and PT (Prothrombin Time) levels abnormal?

Understanding Your Mildly Elevated Coagulation Tests

Your results show mild, isolated prolongation of APTT (43.4 seconds) with borderline elevated PT/INR (15.3 seconds, INR 1.4), which most commonly indicates either lupus anticoagulant, mild factor deficiency, or early anticoagulant effect—but requires a mixing study to differentiate between these causes. 1

What These Numbers Mean

Your coagulation tests show:

• APTT of 43.4 seconds: Mildly prolonged (normal is typically 25-35 seconds depending on the laboratory) 1, 2
• INR of 1.4: Slightly elevated (normal is 0.8-1.2) 3
• PT of 15.3 seconds: Mildly prolonged (normal is typically 11-13.5 seconds) 3

The pattern of isolated or predominantly APTT prolongation with normal or near-normal PT/INR indicates a problem in the intrinsic coagulation pathway, specifically involving factors VIII, IX, XI, XII, or the presence of an inhibitor like lupus anticoagulant. 1, 2

Most Likely Causes in Order of Probability

1. Lupus Anticoagulant (Most Common)

• This is the most frequent cause of isolated APTT prolongation in adults and paradoxically increases clotting risk rather than bleeding risk. 1, 2
• Despite the name, lupus anticoagulant can occur without lupus and is an antiphospholipid antibody. 4
• The baseline PT in patients with lupus anticoagulant is rarely prolonged because phospholipid concentrations in PT reagents are much higher than in APTT reagents, which "quenches" the lupus anticoagulant activity. 4
• Your borderline INR of 1.4 could represent mild lupus anticoagulant effect on the PT reagent used by your laboratory. 4

2. Anticoagulant Medication Effect

• Unfractionated heparin causes APTT prolongation and can mildly affect PT/INR. 1, 2
• Direct oral anticoagulants (DOACs) like rivaroxaban or apixaban can prolong both tests, though the PT is typically more sensitive to factor Xa inhibitors. 4
• Even low-dose heparin from IV line flushes or heparin contamination of the blood sample can cause these findings. 5, 2

3. Mild Factor Deficiencies

• Factor VIII deficiency (hemophilia A) or Factor IX deficiency (hemophilia B) cause isolated APTT prolongation. 1
• Acquired hemophilia (anti-Factor VIII antibodies) is rare but serious, causing bleeding risk. 1
• Mild deficiencies of factors XI or XII can prolong APTT without bleeding risk. 2

4. Laboratory or Technical Issues

• Elevated factor VIII (acute phase reactant) can cause false-negative APTT results, masking underlying issues. 5, 2
• Incorrect blood-to-anticoagulant ratio in the collection tube affects results. 2
• Different APTT reagents have considerable variability in sensitivity to factor deficiencies. 2

Critical Next Step: The Mixing Study

You must have a mixing study performed immediately to differentiate between factor deficiency (corrects) and inhibitor (doesn't correct). 1

The mixing study protocol:

• Mix your plasma 1:1 with normal pooled plasma 1
• Measure APTT immediately and after 1-2 hour incubation at 37°C 1
• Immediate correction suggests factor deficiency 1
• No correction suggests lupus anticoagulant or other inhibitor 1
• Delayed prolongation suggests factor VIII inhibitor (time/temperature-dependent antibodies) 1

Important Clinical Considerations

Bleeding Risk Assessment

• PT/INR of 1.4 does NOT predict increased bleeding risk in patients not on vitamin K antagonists. 6
• The INR was specifically designed and validated only for monitoring warfarin therapy, not as a general bleeding predictor. 6, 7
• A systematic review found weak or no association between pre-procedural INR and bleeding in 78 out of 79 studies. 6
• Your clinical history of spontaneous bleeding or bruising is more important than these numbers for assessing true bleeding risk. 4

Common Pitfalls to Avoid

• Do not assume these results mean you have a bleeding disorder—lupus anticoagulant actually increases clotting risk. 4, 1
• Do not delay necessary procedures solely based on INR 1.4, as this lacks evidence for bleeding prediction. 6
• Do not accept "INR 1.4" as sufficient explanation—the underlying cause must be identified. 1, 2
• In trauma settings, PT/INR is more reliable than APTT for detecting low coagulation factor levels (84% vs 50% sensitivity). 5

What You Should Do Now

Request the following from your physician:

• Mixing study to determine if this is an inhibitor or factor deficiency 1
• Medication review for any anticoagulants, including aspirin, heparin, or DOACs 1
• Lupus anticoagulant panel if mixing study shows no correction 1, 2
• Factor VIII and IX levels if mixing study shows correction 1
• Repeat testing to confirm results weren't due to sample collection issues 2

If acquired hemophilia is identified (rare but serious), this requires immediate dual therapy: bleeding control with bypassing agents and inhibitor eradication with immunosuppression. 1