Abdul Mannan: APTT Mixing Studies Confuse a Lot of People
Abdul Mannan, Consultant Haematologist at Betsi Cadwaladr University Health Board, shared on LinkedIn:
”APTT mixing studies confuse a lot of people.
Let me show you why the incubated step matters.
Factor VIII is thermally labile. At 37°C, it naturally decays over 1-2 hours.
This isn’t pathology—it’s normal biology.
But here’s the trick: this can look exactly like a time-dependent inhibitor if you’re not careful.
The solution? Run parallel tests.
Here’s what you need to know:
- Test both immediate AND incubated samples (at 2 hours)
- Compare patient plasma alone vs 1:1 mix with normal pooled plasma
- Normal thermal decay: slight drift in both NPP and 1:1 mix
- Time-dependent inhibitor: the 1:1 mix prolongs MORE than NPP control
- Golden rule: If Δ(1:1 Mix) > Δ(NPP Control), think inhibitor
The pattern tells the story.
Normal FVIII lability shows similar small shifts in both tubes.
A true inhibitor shows clearly more prolongation in the mix than the control—it’s time AND temperature dependent, not just thermal drift.
Pro tip: Always use properly buffered conditions (Nijmegen-modified) for stability.
Thaw plasma promptly at 37°C and test without delay for routine assays. If you’re uncertain, measure
FVIII:C before and after incubation.
What’s the most challenging APTT mixing study pattern you’ve interpreted recently?”
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