Joan Chapman sent this question. Our institution corrects for HCT greater than 55% for routine and special coagulation assays. It has been our observation, however, that occasionally a patient sample will have a normal PT and PTT in spite of a high crit. We have had a policy that states if the PT and PTT are normal there is no need for anticoagulant correction. Now that is being questioned. CLSI does not address this issue. Do you have any thoughts on this?
Hello, Joan, and thank you for your question. I contend that your policy is sound. A 2006 article compares specimens from high HCT specimens collected in adjusted and un-adjusted anticoagulant volumes; Marlar RA, Potts RM, Marlar AA. Effect on routine and special coagulation testing values of citrate anticoagulant adjustment in patients with high hematocrit values. Am J Clin Pathol. 2006;126:400–5. As you know, failure to adjust the anticoagulant volume causes a relative decrease of the final anticoagulant/plasma ratio, typically prolonging the PT and PTT. The article confirms artifactual PT and PTT prolongation when the anticoagulant is not adjusted, however no article on this subject addresses the over-anticoagulated specimen whose results remain normal. I see no problem with reporting a normal result in this instance.
In the interest of full disclosure, we've recently hosted a provocative discussion about shortened PTT results. While I've held that shortened results are generally caused by collection errors or artifacts, several respected "clotters" cited valid instances where short PTTs held clinical significance. You could miss an otherwise shortened PTT by failing to adjust the anticoagulant, however the likelihood of this is too small to update your policy. Check back here for feedback from our participants.