aPTT Ratio Calculator
aPTT Ratio Calculator
Express a patient aPTT against your laboratory's mean normal as a ratio — the only form in which the result travels between laboratories.
aPTT ratio
Patient ÷ controlPatient aPTT 46 seconds, mean normal aPTT 32 seconds
Formula
- patient aPTT
- the clotting time reported for this sample, in seconds
- mean normal aPTT
- the geometric mean of the laboratory's own reference population on the current reagent and analyser lot — not the upper limit of the reference range, which gives a ratio biased low
- why a ratio
- absolute seconds depend on the reagent's phospholipid and activator, and on the analyser, so they are not comparable between laboratories; the ratio removes most of that difference
Worked example
Patient aPTT 46 seconds, mean normal aPTT 32 seconds
46 ÷ 32 = 1.44
Between 1.2 and 1.5 → mildly prolonged
Confirm on a repeat sample, then consider a 1:1 mix to separate factor deficiency from an inhibitor
Reading the ratio
| Ratio | Interpretation | Next step |
|---|---|---|
| < 0.8 | Shortened | Almost always pre-analytical — repeat on a cleanly collected, correctly filled citrate tube |
| 0.8 – 1.2 | Normal | No intrinsic pathway abnormality demonstrated by this test |
| 1.2 – 1.5 | Mildly prolonged | Repeat, check for heparin, then perform a 1:1 mix |
| 1.5 – 2.5 | Prolonged | Traditional unfractionated heparin band, if locally calibrated; otherwise investigate |
| > 2.5 | Markedly prolonged | Exclude line contamination with heparin, then over-anticoagulation, severe deficiency, inhibitor or DIC |
Prolonged aPTT: the 1:1 mixing study
| Result of 1:1 mix with normal plasma | Suggests | Typical causes |
|---|---|---|
| Corrects to normal | Factor deficiency | Haemophilia A or B, factor XI or XII deficiency, von Willebrand disease, liver disease, vitamin K deficiency |
| Does not correct | Inhibitor | Lupus anticoagulant most often; less commonly an acquired factor VIII inhibitor, which is time and temperature dependent |
| Corrects, then prolongs on incubation | Time-dependent inhibitor | Acquired factor VIII inhibitor — incubate the mix at 37°C for one to two hours before repeating |
Why the ratio, and what a prolonged one means
The aPTT ratio exists because absolute seconds do not travel. The clotting time depends on the phospholipid composition and contact activator in the reagent and on the analyser’s endpoint detection, so a normal aPTT can be 26 seconds in one laboratory and 36 in another. Dividing by the laboratory’s own mean normal removes most of that difference and produces a number that means roughly the same thing wherever it is generated. The denominator should be the mean of the local normal population, not the upper limit of the reference range, which biases the ratio downwards.
The traditional therapeutic band for unfractionated heparin, roughly 1.5 to 2.5 times control, needs a caveat attached to it. Reagent sensitivity to heparin varies widely, so the same anti-Xa level can give quite different aPTT ratios on different reagents, and a band copied from a textbook may be therapeutic on one analyser and sub-therapeutic on another. Each laboratory should establish its own band by calibrating the aPTT against anti-Xa on the reagent in use, and increasingly anti-Xa is measured directly instead.
When a prolonged ratio is not explained by heparin, the 1:1 mixing study is the first discriminating step. Mixing patient plasma with an equal volume of normal plasma restores at least half of every factor, which is enough to correct the time if the problem is a deficiency. Failure to correct implies something in the plasma is inhibiting the reaction — most often a lupus anticoagulant, occasionally an acquired factor VIII inhibitor, which characteristically corrects initially and then prolongs after incubation at 37°C.
A shortened ratio is rarely a real finding and should not be read as evidence of a hypercoagulable state. It is usually pre-analytical: a traumatic or slow venepuncture that partially activates the sample, a partly clotted tube, or an underfilled citrate tube in which the excess citrate relative to plasma changes the assay conditions. A very high haematocrit has the same effect for the same reason, by reducing the plasma volume the fixed citrate is diluted into. Repeat the sample before drawing any conclusion from it.
Frequently asked questions
How is the aPTT ratio calculated?
Divide the patient’s aPTT in seconds by the laboratory’s mean normal aPTT in seconds. A patient aPTT of 46 seconds against a mean normal of 32 seconds gives a ratio of 1.44.
Why not just use the aPTT in seconds?
Because seconds are reagent- and analyser-specific. A normal aPTT can be 26 seconds in one laboratory and 36 in another, so the absolute value does not transfer. The ratio does.
Is 1.5 to 2.5 the right heparin therapeutic range?
It is the traditional band, but it must be calibrated locally against anti-Xa. Reagents differ substantially in heparin sensitivity, so a band borrowed from elsewhere may be therapeutic on one analyser and sub-therapeutic on another.
What does it mean if a prolonged aPTT corrects on 1:1 mixing?
It suggests a factor deficiency, because adding normal plasma restores enough of the missing factor to normalise the time. Failure to correct suggests an inhibitor, most often a lupus anticoagulant.
Does a short aPTT mean the patient is hypercoagulable?
Usually not. A shortened result is nearly always pre-analytical — a difficult draw, a partly clotted or underfilled citrate tube, or a very high haematocrit. Repeat on a properly collected sample before interpreting it.
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References
- Clinical and Laboratory Standards Institute. Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays. CLSI guideline H21.
- Olson JD, Arkin CF, Brandt JT, et al. College of American Pathologists Conference XXXI on laboratory monitoring of anticoagulant therapy: laboratory monitoring of unfractionated heparin therapy. Arch Pathol Lab Med. 1998;122:782–798.
- Keeling D, Mackie I, Moore GW, Greer IA, Greaves M; British Committee for Standards in Haematology. Guidelines on the investigation and management of antiphospholipid syndrome. Br J Haematol. 2012;157:47–58.