What causes prolonged PT, elevated INR, and elevated aPTT without anticoagulant use?

Mildly Prolonged PT/INR and aPTT Without Anticoagulant Use

Your coagulation values show mild prolongations that are most likely clinically insignificant, but require systematic evaluation to exclude acquired factor deficiencies, lupus anticoagulant, or early vitamin K deficiency—particularly if you have any bleeding symptoms, recent illness, malabsorption, or dietary restrictions.

Initial Assessment Priority

Your laboratory pattern shows:

• PT 15.3 seconds (mildly prolonged, typically normal <14.5 seconds) 1
• INR 1.4 (mildly elevated, normal <1.2)
• aPTT 43.4 seconds (prolonged, typically normal <35-40 seconds depending on laboratory)
• Platelets 240 (normal) 1
• Normal liver function (excludes hepatic synthetic dysfunction)

This pattern indicates involvement of both the intrinsic and common coagulation pathways, since both PT and aPTT are prolonged. 1, 2

Most Likely Causes in Your Clinical Context

1. Lupus Anticoagulant (Most Common in Asymptomatic Patients)

• Lupus anticoagulant paradoxically prolongs PT and especially aPTT in vitro but causes thrombosis, not bleeding, in vivo. 1, 2
• This is the most common cause of isolated prolonged aPTT in asymptomatic patients without bleeding 2
• Associated with antiphospholipid syndrome, which increases risk of venous/arterial thrombosis and pregnancy loss 2
• Critical distinction: Normal platelets argue against this being clinically significant coagulopathy 1

2. Mild Vitamin K Deficiency (If Any Risk Factors Present)

• Vitamin K deficiency affects factors II, VII, IX, and X simultaneously, prolonging both PT and aPTT 3, 2
• Risk factors to assess: malabsorption (celiac disease, inflammatory bowel disease, chronic diarrhea), recent antibiotic use (disrupts gut flora), poor dietary intake, bile duct obstruction 3
• Your normal liver function makes severe deficiency unlikely, but subclinical deficiency remains possible 1

3. Factor XII Deficiency (Clinically Insignificant)

• Factor XII deficiency prolongs aPTT but does NOT cause bleeding 1, 4
• This is a laboratory finding without clinical consequences 1
• No treatment needed even if confirmed

4. Mild Acquired Factor Deficiencies

• Hypothermia, acidosis, or recent illness can transiently prolong coagulation tests 1
• Your values are only mildly prolonged, making severe deficiency unlikely 1

Diagnostic Algorithm

Step 1: Clinical Context Assessment

Determine if you have:

• Any bleeding symptoms (easy bruising, gum bleeding, heavy menstrual bleeding, nosebleeds) 1, 5
• Recent illness, infection, or COVID-19 (can cause transient coagulopathy) 1
• Malabsorption symptoms (chronic diarrhea, weight loss, steatorrhea) suggesting vitamin K deficiency 3
• History of thrombosis or pregnancy loss (suggests antiphospholipid syndrome) 2

Step 2: Mixing Study

• A 1:1 mixing study with normal pooled plasma distinguishes factor deficiency from inhibitors 4, 5
• If the mix corrects (PT/aPTT normalize): Factor deficiency is present 4
• If the mix does NOT correct: Inhibitor present (lupus anticoagulant or acquired factor inhibitor) 4, 2

Step 3: Specific Testing Based on Mixing Study

If mixing study corrects (factor deficiency):

• Measure individual factor levels: II, V, VII, VIII, IX, X, XI 5, 2
• Check vitamin K level or trial of vitamin K 10mg orally for 3 days, then repeat PT/aPTT 3
• If PT/aPTT normalize after vitamin K, diagnosis confirmed 3

If mixing study does NOT correct (inhibitor):

• Lupus anticoagulant panel (dilute Russell viper venom time, hexagonal phase phospholipid neutralization) 2
• Anticardiolipin antibodies and anti-β2-glycoprotein I antibodies 2
• If lupus anticoagulant confirmed, assess thrombosis risk, NOT bleeding risk 2

Critical Pitfalls to Avoid

Common Misinterpretations

• INR is NOT the same as PT ratio and can miss subtle prolongations 1
• Mildly prolonged PT/aPTT does NOT automatically indicate bleeding risk 1
• Normal platelet count (240) makes significant bleeding disorder very unlikely 1

Pre-analytical Errors

• Specimen handling errors commonly cause false-positive results 1
• Heparin contamination from central line draws artificially prolongs tests 1
• Ensure blood was drawn peripherally, not from IV line 1

When to Worry

You should pursue urgent evaluation if:

• PT >1.5 times normal (>18 seconds) or INR >1.8 1
• aPTT >1.5 times normal (>60 seconds) 1
• Any active bleeding 1
• Platelet count drops below 150 1
• Fibrinogen <2.0 g/L if measured 1

Recommended Next Steps

For your specific values (PT 15.3, INR 1.4, aPTT 43.4):

1. Repeat testing peripherally to exclude specimen error 1
2. Perform mixing study to differentiate factor deficiency from inhibitor 4, 5
3. If you have ANY bleeding symptoms, measure specific factor levels and consider hematology consultation 1, 5
4. If completely asymptomatic, lupus anticoagulant testing is most appropriate next step 2
5. Consider empiric vitamin K 10mg orally if any malabsorption risk factors, then recheck in 3 days 3

Your normal platelet count and liver function are reassuring and make life-threatening coagulopathy extremely unlikely. 1 However, the pattern warrants systematic evaluation to identify the specific cause and determine if any intervention or monitoring is needed.