Increased packed cell volume alters point of care viscoelastic clotting parameters in horses.
Authors: Bishop Rebecca C, Jonk Kaitlyn M, Migliorisi Alessandro, Austin Scott M, Mullins Emma C, Wilkins Pamela A
Journal: Equine veterinary journal
Summary
# Editorial Summary Polycythaemia and coagulopathy both independently worsen survival odds in critically ill horses, yet little has been known about their interaction when they occur together. Since viscoelastic coagulation monitoring (VCM Vet™) has become increasingly available for point-of-care assessment, Bishop and colleagues sought to determine whether elevated packed cell volume might artefactually alter these measurements in seven healthy mares given intravenous phenylephrine to induce transient polycythaemia. The researchers found that PCV rose from a baseline of 37% to 56.3% within 5 minutes, and this acute increase produced significantly reduced clot amplitudes at 10 and 20 minutes (A10 and A20 dropped from 12.6 and 18.9 units to 5.4 and 7.6 respectively)—changes consistent with hypocoagulability—yet conventional plasma-based tests (PT and PTT) remained normal throughout. Strong negative correlations emerged between PCV and the VCM parameters A10 (R = −0.9), A20 (R = −0.87) and clot formation time (R = −0.61). For practitioners relying on viscoelastic testing to assess critical patients at risk of thrombosis or coagulopathy, this research signals a critical confound: high PCV alone can produce false hypocoagulable traces without true platelet or coagulation dysfunction, potentially leading to misinterpretation of clotting status and inappropriate therapeutic decisions in polycythaemic horses.
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Practical Takeaways
- •Increased PCV from any cause (polycythaemia, dehydration, splenic contraction) can artificially worsen viscoelastic coagulation parameters (VCM Vet) without true coagulopathy; interpret results cautiously in critical cases with concurrent polycythaemia
- •Plasma-based coagulation tests (PT, PTT) appear unaffected by PCV changes and may be more reliable for true coagulation assessment in polycythaemic horses, whereas viscoelastic testing may overestimate clotting deficits
- •When evaluating critically ill horses with both polycythaemia and suspected coagulopathy, use multiple coagulation assessment methods and correct PCV abnormalities before drawing conclusions about true clotting dysfunction
Key Findings
- •Phenylephrine-induced polycythaemia increased PCV from 37% to 56.3% at T1, causing a significant decrease in VCM Vet amplitude parameters (A10 and A20) suggesting hypocoagulability
- •PCV showed strong negative correlations with clot formation time (CFT, R=-0.61), A10 (R=-0.9), and A20 (R=-0.87)
- •Plasma-based coagulation tests (PT and PTT) remained within normal ranges and showed no significant changes despite polycythaemia, indicating viscoelastic testing may be more sensitive to PCV changes
- •Splenic volume decreased from 11.5L to 6.1L at T1 following phenylephrine administration, confirming transient polycythaemia induction mechanism