The order of draw, memorized.
The single most-tested ASCP PBT concept. Six tubes, one strict sequence, one mnemonic that survives test-day nerves. Twelve minutes from here to confident.
Why this matters
Order-of-draw violations are one of the top three sources of pre-analytical error in clinical labs. The ASCP BOC PBT content guideline assigns roughly half of every exam to specimen collection, and within that, tube sequence is the most directly testable single fact. Get this lesson right and you have already secured a meaningful slice of your score before you study anything else.
Key takeaways
- Yellow before everything. Blood culture (SPS) tubes draw first to keep the sample sterile. Drawing anything else first risks contamination from the prior tube's additive, which can give a false-positive culture.
- Blue after Yellow. Light-blue sodium-citrate tubes for coagulation panels (PT, INR, PTT) draw second. These need an exact 9:1 fill ratio — under-fill and the test is rejected.
- Red/Gold next. Plain serum or SST gel tubes for chemistry. SST tubes inverted gently 5 times; never shake.
- Green for stat chem. Lithium- or sodium-heparin tubes for stat electrolytes and ammonia. Glucose belongs on Gray, not Green — a common test-day trap.
- Lavender for hematology. EDTA tubes for CBC, blood type, HbA1c. Fill exactly to the line; under-fill skews CBC indices.
- Gray last. Sodium-fluoride / potassium-oxalate for fasting glucose and lactate. Fluoride preserves glucose by inhibiting glycolysis — that's why glucose belongs here, not under Green.
The mnemonic the ASCP examiners themselves see students use most: Yellow Boys Run Green Lavender Gray. Six letters, six stops, in order. Recite it forward and backward before test day. The exam will give you a tube sequence and ask you to identify the violation, which is the same skill in reverse.
Try the question your examiner is most likely to ask
A nurse asks you to redraw a chemistry panel because the prior potassium resulted at 7.2 mEq/L on a patient with no cardiac or renal history. The previous draw sequence was: gold SST, then lavender EDTA, then green lithium heparin. What is the most likely pre-analytical cause?
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Standards reference: CLSI GP41-Ed7, Collection of Diagnostic Venous Blood Specimens, §6.4 (Order of Draw). Tube-additive interactions cross-referenced against the ASCP BOC PBT content guideline, domain "Specimen Collection." PhlebotomySkills.com is exam-preparation content. Not a degree, not for-credit coursework, and not affiliated with any certifying body.
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