Also called: Coagulation analyzer, Coagulometer, PT/INR machine

Coagulation Analysers: PT/INR, APTT and Choosing a Coagulometer

Clotting tests run on different machines, from different tubes, under stricter sample rules than the FBC. What each test measures and which kind of analyser fits a clinic, an anticoagulation service or a hospital lab.

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Last updated: 9 October 2026

What a coagulation analyser measures

A haematology analyser counts cells; a coagulation analyser measures how long plasma takes to clot, or how much of a clotting-related protein it contains. The FBC machine cannot do these tests.

Test	What it reflects	Common use
PT and INR	The extrinsic and common clotting pathways	Monitoring warfarin; liver function; pre-procedure screening
APTT	The intrinsic and common pathways	Monitoring unfractionated heparin; investigating bleeding
Fibrinogen	The amount of fibrinogen, usually by the Clauss method	Bleeding, liver disease, suspected DIC
D-dimer	Breakdown products of cross-linked fibrin, by immunoassay	Helping to exclude venous thrombosis in low-risk patients; DIC

The citrate sample: stricter than the FBC

Light-blue tube, 3.2% citrate

Coagulation samples are taken into a light-blue-top tube containing 3.2% (0.109 mol/L) sodium citrate, which binds calcium. The analyser adds calcium back to start the clot.

The 9:1 ratio

Nine parts blood to one part citrate is critical. An under-filled tube leaves too much citrate and falsely prolongs clotting times, so labs reject tubes below the fill mark. Very high haematocrits also need an adjusted citrate volume — follow your lab’s procedure.

Clean draw, gentle mixing

Avoid a traumatic draw and prolonged tourniquet time, invert gently a few times, and with a winged (butterfly) set draw a discard tube first so the air in the tubing does not under-fill the citrate tube. Clotted or haemolysed samples are rejected.

Platelet-poor plasma and timing

Most tests are run on platelet-poor plasma after centrifugation, as your procedure specifies. Keep samples at room temperature rather than refrigerating whole blood, and test within the time limits your lab has validated — APTT for heparin monitoring is the most time-sensitive.

Three kinds of coagulation analyser

	POC INR meter	Semi-automated benchtop	Fully automated analyser
Sample	Finger-prick whole blood on a strip	Citrated plasma, pipetted by the operator	Citrated plasma, sampled from the tube (some pierce the cap)
Tests	PT/INR only	PT, APTT, fibrinogen; some add more	Full menu including D-dimer and factor assays on many models
Throughput	One patient at a time	Low — one to four channels	Tens to hundreds of tests an hour
Needs a centrifuge	No	Yes	Yes
Typical user	Warfarin clinics, dental and GP clinics, patient self-testing	Small hospitals and private labs	Hospital and reference laboratories

Clot detection: mechanical or optical

Mechanical

A steel ball or similar sensor detects the rise in viscosity as the clot forms. It is less affected by cloudy, icteric or haemolysed plasma.

Optical

A light beam detects the change in turbidity as fibrin forms. Optical analysers can also run chromogenic and immunoturbidimetric assays such as D-dimer on the same platform, but very lipaemic or icteric samples can interfere.

INR and quality control

The INR corrects the PT for the sensitivity of the reagent and instrument (the ISI), so warfarin results can be compared between labs.
Each new PT reagent lot needs its ISI and the local mean normal PT set or verified before INRs are reported.
Run normal and abnormal controls at the frequency your procedure sets — typically each day of testing and after reagent changes.
POC INR meters need their own control checks and periodic comparison with a laboratory INR, and are less reliable at high INRs or outside their haematocrit range.
Take part in a coagulation EQA scheme if you report results for patient management.

Which one does your site need?

GP and dental clinics

Most send coagulation tests to an outside lab. A POC INR meter makes sense if you regularly check INR in patients on warfarin before procedures or adjust their dose.

Anticoagulation services

POC INR meters give a result within minutes during the visit, which makes dose adjustment in one appointment possible.

Hospitals and labs

A semi-automated analyser suits low volumes; a fully automated analyser is needed once PT, APTT, fibrinogen and D-dimer run throughout the day and night.

Registration in Malaysia

Haematology analysers and their reagents are in-vitro diagnostic (IVD) medical devices. Under the Medical Device Act 2012 (Act 737), a medical device must be registered with the Medical Device Authority (MDA) before it is imported, exported or placed on the Malaysian market — so ask for the registration number of the analyser and its reagents and check them on the MDA’s public register before you buy.

Hematology Analyser supply near you

Delivery, installation and training across Malaysia from Batu Caves, Kota Kinabalu and Kuching.

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Frequently asked questions

Can a haematology analyser do PT and APTT?

No. PT, APTT and other clotting tests need a coagulation analyser and a citrate tube; the FBC machine counts cells from an EDTA tube.

What is the difference between PT and INR?

PT is the clotting time in seconds. The INR converts it into a ratio corrected for the reagent’s sensitivity, so results can be compared between labs — it is the number used to manage warfarin.

Why was my coagulation sample rejected for under-filling?

The citrate tube must keep a 9:1 ratio of blood to citrate. Too little blood leaves excess citrate, which falsely prolongs the clotting times.

Do patients on newer oral anticoagulants need INR checks?

Routine INR monitoring is used for warfarin. Direct oral anticoagulants are not monitored with the INR in routine care, and the INR does not reliably show their effect — follow the prescriber’s and the lab’s guidance.

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