Medically reviewed by Dr. Hafiz Muhammad Haseeb, M.B.B.S., F.C.P.S. (Haematology), Consultant Haematologist, Al-Nasar Lab & Diagnostic Centre. Updated September 2026.
Quick answer: A coagulation profile measures how long your blood takes to clot. PT and INR assess one clotting pathway and are used to monitor warfarin. APTT assesses the other and is used for heparin and to investigate bleeding disorders. For most people on warfarin the INR target is 2.0 to 3.0, and it is checked regularly because diet, illness and antibiotics all shift it.
Book a coagulation profile: call 0300 0341944, or arrange home sample collection across Lahore on 0309 1166944.
What do the individual tests measure?
| Test | What it assesses | Typical reference |
|---|---|---|
| PT (Prothrombin Time) | The extrinsic and common pathway. Sensitive to liver function and vitamin K | roughly 11 to 13.5 seconds |
| INR | PT standardised so results are comparable between laboratories | 0.8 to 1.2 if not on warfarin |
| APTT | The intrinsic and common pathway. Used for heparin monitoring and haemophilia | roughly 25 to 35 seconds |
| Fibrinogen | The protein that forms the clot itself | roughly 200 to 400 mg/dL |
| D-dimer | A breakdown product of clot. Used mainly to help exclude clots | varies by assay |
| Platelet count | From the blood count. Platelets do the first stage of clotting | 150,000 to 400,000 per microlitre |
Ranges differ between laboratories and reagents, so read the range printed on your own report.
Why does INR exist when PT already measures the same thing?
Because PT results genuinely differ between laboratories. The chemical reagents used vary in sensitivity, so the same blood sample can produce meaningfully different PT values in two different laboratories.
That was dangerous for warfarin patients, where dose decisions depend on the number. The INR applies a correction factor for the specific reagent used, producing a standardised result.
The practical consequence: an INR of 2.5 means the same thing anywhere in the world, whereas a PT of 25 seconds does not. This is why warfarin is always managed by INR and never by raw PT.
What is the INR target?
- Not on anticoagulants: roughly 0.8 to 1.2
- Atrial fibrillation, DVT, pulmonary embolism: usually 2.0 to 3.0
- Mechanical heart valves: usually higher, often 2.5 to 3.5, depending on the valve and its position
Your cardiologist or physician sets your personal target. Do not adopt a range from a website or from another patient.
The two directions of risk are different but both serious. An INR below target means the blood is clotting too readily, raising the risk of stroke or clot. An INR above target means bleeding risk, and a markedly high INR needs urgent medical attention, particularly with any bleeding, a fall or a head injury.
What makes INR swing?
This is where most patients run into trouble, because warfarin interacts with a great deal.
Raises INR, meaning more bleeding risk:
- Antibiotics, especially ciprofloxacin, metronidazole and co-trimoxazole. This is a very common cause of a sudden high INR
- Antifungal medicines
- Aspirin and other NSAID painkillers
- Alcohol, particularly binge drinking
- Liver disease, fever, diarrhoea and poor food intake
- Thyroid overactivity
Lowers INR, meaning more clotting risk:
- Vitamin K rich foods eaten in larger amounts than usual: spinach, saag, broccoli, cabbage, green leafy vegetables
- Rifampicin, carbamazepine and some other medicines
- Missed doses
An important clarification about diet, because it is frequently misunderstood. You do not need to avoid green vegetables. What matters is consistency. A steady moderate intake allows your dose to be set correctly around it. The problem is sudden change, such as eating a large amount of saag one week and none the next.
Why must the sample tube be filled exactly?
Coagulation testing has a technical requirement most patients never hear about, and it causes more rejected samples than any other test.
The tube contains a measured amount of anticoagulant, and the ratio of blood to that anticoagulant must be exact, at nine parts blood to one part additive. That is why the tube has a fill line.
An underfilled tube produces a falsely prolonged PT and INR, because there is proportionally too much anticoagulant. Acting on such a result could lead to an unnecessary warfarin dose reduction and a real clot risk.
If the phlebotomist says the sample must be redrawn, this is usually why. It is a safety step, not an inconvenience.
Two related points: the sample should be tested promptly, and a very high haematocrit can also affect the ratio and requires tube adjustment.
When is a coagulation profile needed?
- Monitoring warfarin, the most common reason. Frequency varies from weekly when starting to monthly once stable
- Before surgery or any invasive procedure
- Investigating easy bruising, frequent nosebleeds, heavy periods or prolonged bleeding from small cuts
- Suspected haemophilia or von Willebrand disease, particularly with a family history
- Assessing liver disease, since the liver makes most clotting factors. PT is one of the better indicators of liver synthetic function, alongside liver function tests
- Suspected vitamin K deficiency, including in newborns
- Investigating recurrent miscarriage or unexplained clots
- In serious illness, where clotting can become disordered
How to prepare
- No fasting is usually required
- Bring a complete list of your medicines, including herbal products and supplements. Many interact with warfarin
- Mention any recent antibiotic course, which is one of the most common reasons an INR jumps
- Take warfarin at the same time each day, usually evening, and keep that routine on test days
- Mention any recent illness, fever, vomiting or diarrhoea
- Use the same laboratory where possible for consistency
- Bring your anticoagulation record book so the trend is visible
Coagulation testing at Al-Nasar Lab
Al-Nasar Lab & Diagnostic Centre has provided laboratory testing in Lahore since 1996 and holds ISO 15189:2022 accreditation from the Pakistan National Accreditation Council. PT, INR, APTT, fibrinogen, D-dimer and complete blood count are all available, with home sample collection across Lahore and same day reports.
Home collection is particularly useful for patients on long term warfarin, for whom regular monitoring otherwise means repeated trips. See our accreditations, find your nearest collection centre, or contact us to book.
Frequently Asked Questions
What is a normal INR?
Roughly 0.8 to 1.2 if you are not on anticoagulants. On warfarin the target is usually 2.0 to 3.0, or higher for some mechanical heart valves.
Do I need to fast for a PT INR test?
Usually not. What matters far more is telling the laboratory about your medicines, especially any recent antibiotics.
Can I eat green vegetables on warfarin?
Yes. You do not need to avoid them. Keep your intake consistent, because sudden changes in vitamin K intake are what destabilise the INR.
Why was my blood sample rejected?
Most often because the tube was not filled to the line. Coagulation tests need an exact ratio of blood to anticoagulant, and an underfilled tube gives a falsely high INR.
What is the difference between PT and APTT?
They assess different clotting pathways. PT and INR are used for warfarin and liver function, while APTT is used for heparin and for investigating haemophilia.
This article is for general information and is not a substitute for medical advice. Never adjust your warfarin dose yourself. Discuss all results with the doctor managing your anticoagulation.
