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Within the serum category, the preferred sequence is plain red (no additive) BEFORE the serum separator tube (SST) containing clot activator and gel. Drawing the SST first increases the chance of clot activator carryover into the plain red tube, potentially altering downstream serology or special chemistry assays validated on additive-free serum. While both tubes ultimately yield serum, additive profiles differ and matter analytically. EDTA (Option D) must never precede serum because EDTA carryover chelates calcium and can distort chemistry/immunoassay results. “Either order” (Option C) overlooks additive interaction risks. After draw, allow complete clotting (≈30 minutes for standard serum; use rapid thrombin tubes only if validated), then centrifuge per manufacturer RCF/time. Proper micro-ordering within tube families is an often-tested nuance that prevents subtle, hard-to-detect preanalytical errors.