Short Draws

This 12/23/16 “short draw” question from Adam McMullen is copied from MEDLAB-L, hosted by Pat LeTendre. Adam and Pat have granted permission for its reproduction here.

Hello, We use BD Vacutainer® 2.7 mL  for routine coagulation testing–PT/INR, PTT, and fibrinogen. These tubes provide a minimum fill line. Our policy requires we adhere to this line, and if filled to below the line, we must cancel the test. Does any lab out there allow a little discretion on this minimum fill line in terms of specimen acceptance? For instance, if there is a very small gap between the fill line and plasma, would any lab allow techs to use discretion in these situations and use this tube for testing? Or is everyone 100% strict with this minimum fill line?

I can see both sides to this. Strict adherence allows for better results. Allowing a little discretion means not re-sticking a sick patient or dragging an outpatient in for another blood collection over a few microliters of blood that probably would not yield a clinically different result. Any input is greatly appreciated.

Here is a response from colleague, friend, and Fritsma Factor technical advisor, David McGlasson, also reproduced from MEDLAB-L with Dave’s permission:

Adam, In McGlasson DL, Kaczor DA, Krasuski RA, Campbell CL, Kostur MR, Adinaro JT. Effects of pre-analytical variables on the anti-activated factor X chromogenic assay when monitoring unfractionated heparin and low molecular weight heparin anticoagulation. Blood Coag Fibrinolysis 2005; 16:173–6; we drew a “short draw” of 6:1 blood to anticoagulant (AC) ratio and a standard 9:1 blood/AC ratio and compared six variables, including PTTs and anti-Xa chromogenic assays. The short draw affected the PTTs but had no effect on the chromogenic assays.

The problem with accepting the short draw is if you must perform additional clottable assays such as lupus anticoagulant profiles or factor assays. In this case, you might be giving out incorrect data. Remember in lab work it may be garbage in and garbage out (GIGO) when it comes to giving the physician the information needed to treat the patient. Hope this helps.

This follow-up comment from Jane Bossart, Tri City Medical Center, appeared in MEDLAB-L on 12/30/16:

Keep in mind that an underfilled tube may give a falsely prolonged result because there is too much AC for the amount of plasma. However, consider that a specimen with a reduced HCT counteracts this effect because it supplies relatively more plasma. Also keep in mind that the fill line on the tube is already at the minimum acceptable level.

That being said, we sometimes do allow some discretion especially on specimens from the ED, considering that these are essentially screens. We will run a slightly underfilled tube and if the result is
normal we result it. If it is abnormal, or if the patient is on anticoagulant therapy, it is recollected. It is surprising how many underfilled tubes yield normal results. The 3.2% citrate is truly more forgiving than the 3.8% that was used many years ago.

Along the same lines, if a specimen has a HCT greater than 55%, we also test and report normal results. A high HCT has the same effect as an underfilled tube: excess anticoagulant.

George invites additional discussion on this subject. Since 2003, BD Vacutainer® has provided maximum and minimum fill indcations for their 3.2% sodium citrate tubes based on their extensive research. They cite CLSI document H21, which states, “Unless the package insert indicates otherwise, or in-house studies were performed, 90% fill volume is recommended.” This statement cites Adcock D, Kressin DC, Marlar RA. Minimum specimen volume requirements for routine coagulation testing. Am J Clin Pathol 1998;109:595–9, and Chang J, Sadler MA, Witt DM. Impact of evacuated tube fill volume and mixing on routine coagulation measures using 2.5 mL pediatric tubes. Chest 2004;126:1262–6. Most studies on 3.2% sodium citrate evacuated tube fill volumes helped establish the 90% rule and also confirm that 3.2% citrate is more “forgiving” than 3.8%, largely discontinued in 2001. These studies largely employed the “routine” clot-based assays PT/INR and PTT, so as Dave McGlasson indicates, advanced coagulation studies like factor assays and lupus anticoagulant profiles must be performed on full-draw, correctly collected specimens. I hope this discussion answers Adam’s question.