Is the classic order of draw actually grounded in additive carryover data, or is it mostly convention that stuck? I get the logic about EDTA contaminating calcium dependent assays and how a speck in a light blue tube can skew coag, but I’ve never seen more than manufacturer notes and textbooks repeating each other. I’ve worked in two labs that swapped the SST and plain red without drama, and a third that insisted on SST after red, so the consistency seems fuzzy. Not trying to argue, I just want to know if there’s a primary study or guideline that measured carryover between specific tubes under typical inversions and routine draw sequences. If you’ve got a link or citation that nails it down, I’d appreciate it.
Ask a clinical pathologist or MLT, someone who actually tests the specimens
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Short answer: it’s both. There’s actual data for the big ones—EDTA carryover depressing Ca/Mg/K, citrate being touchy for coag, and silica from serum tubes messing with coag if it carries over. CLSI’s guidance (GP41, ex-H3) leans on small carryover studies and manufacturer testing, though a lot of those are older and not with the newest closed systems. More recent work suggests that with modern evacuated tubes and clean technique, most pairs don’t show meaningful bias, but EDTA and citrate remain the problem children. Blood cultures first is more about skin flora and antiseptic, not additives.
On a QC review we saw BMP redraws jump whenever lavender sneaked in before serum. K high. Ca low. Fixing order fixed it. Takeaway: there’s solid data for the big ones (EDTA affecting Ca/K, citrate for coag, some silica).