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Order of draw and tube additives

The exam rarely hands you a tube color. It hands you a test — PT/INR, CBC, basic metabolic panel — and expects you to convert it to a tube and then to a position in the CLSI sequence. A color mnemonic alone does not survive that.

1. Blood culturesYellow SPS bottle or tube, 8–10 inversions. Always first, before any additive can reach the bottle (CLSI PRE02, formerly GP41).
2. Light blueSodium citrate — PT/INR and aPTT. 3–4 inversions, and it must reach the fill line: the 9:1 blood-to-citrate ratio is part of the test.
3. Red or goldClot activator, plus gel in the gold SST. 5 inversions for plastic; a plain glass red tube has no additive and takes 0. Clot upright 30 minutes before centrifuging, and separate serum from cells within 2 hours (CLSI GP44).
4. GreenLithium or sodium heparin, for plasma chemistries. 8–10 inversions.
5. Lavender or pinkK2 EDTA — CBC, HbA1c, type and screen. 8–10 inversions immediately, or the specimen microclots and the platelet count is rejected.
6. GraySodium fluoride with potassium oxalate — glucose and lactate. 8–10 inversions. Last, because fluoride and oxalate interfere with the widest range of other tests.
Fingerstick runs backwardsThe CLSI GP42 dermal order is blood gas, then EDTA, then other additives, then serum. EDTA comes first here — the opposite of venipuncture — because capillary blood starts clotting the moment it reaches the skin.
Butterfly plus light blueWhen a winged set collects a coagulation tube first, draw a discard tube first to fill the tubing dead space, or the citrate tube comes up short.
TourniquetOne minute maximum before the vein is entered. Longer hemoconcentrates the sample and falsely raises potassium, total protein and cell counts (CLSI PRE02).

Where the point is lost: Drawing lavender before the gold serum tube lets K2 EDTA carry over, and the panel comes back with a falsely high potassium and a falsely low calcium — it reads like a real electrolyte emergency. The other trap: an underfilled light blue tube is recollected, never topped off from a second tube and never diluted, because the excess citrate falsely prolongs the PT/INR.

Order of draw and tube additives

12 questions on order of draw, each with an explanation and statute citation.

12 questions

Pass line: 78%, same as the real exam

Questions and answers, explained

All 12 questions above, with the correct answer and why it is correct. Everything here is on order of draw and tube additives.

  1. What is the maximum length of time a tourniquet should remain in place before the vein is entered?

    • A30 seconds
    • B1 minuteCorrect
    • C2 minutes
    • D3 minutes

    Why: CLSI limits tourniquet application to 1 minute before the vein is entered, because longer application drives water and small molecules out of the capillaries and hemoconcentrates the sample, falsely raising potassium, total protein, and cell counts. Two or three minutes (C, D) is well past the point where preanalytical error begins.

    Reference Domain 3E; CLSI venipuncture standard (PRE02, formerly GP41) one-minute tourniquet limit

  2. The medical assistant has had the tourniquet on the patient's arm for about 90 seconds while searching for a vein. What should be done next?

    • AComplete the draw quickly and note the delay on the requisition.
    • BLoosen the tourniquet slightly and keep searching for the vein.
    • CRelease the tourniquet, wait about 2 minutes, then reapply it and perform the draw.Correct
    • DMove the tourniquet 3 inches higher on the arm and continue.

    Why: Once the 1-minute CLSI limit is exceeded, the correct correction is to release the tourniquet and wait about 2 minutes so circulation normalizes before reapplying. Simply loosening it (B) or repositioning it (D) does not reverse the hemoconcentration already developing, and finishing quickly (A) sends a specimen with falsely elevated potassium and protein to the laboratory.

    Reference Domain 3E; CLSI venipuncture standard (PRE02, formerly GP41) tourniquet reapplication after 2 minutes

  3. Following the CLSI order of draw, which tube is collected immediately after blood culture bottles?

    • ALight blue sodium citrate tubeCorrect
    • BGold serum separator tube
    • CLavender EDTA tube
    • DGreen heparin tube

    Why: The CLSI sequence is blood cultures, light blue sodium citrate, red or gold serum, green heparin, lavender EDTA, then gray fluoride/oxalate. The light blue tube is second so its precise 9:1 blood-to-citrate ratio is protected and no additive from another tube carries over into the coagulation specimen. Gray is always last because fluoride and oxalate contaminate the widest range of other tests.

    Reference Domain 3E; k132 order of draw; CLSI order of draw (PRE02, formerly GP41)

  4. A provider orders a PT/INR, a CBC, and a basic metabolic panel (gold serum separator tube) on the same patient. In what order should the tubes be drawn?

    • AGold serum separator, light blue, lavender
    • BLavender, light blue, gold serum separator
    • CLight blue, gold serum separator, lavenderCorrect
    • DLight blue, lavender, gold serum separator

    Why: PT/INR is collected in the light blue citrate tube, the basic metabolic panel in the gold serum separator tube, and the CBC in the lavender EDTA tube, so the CLSI sequence gives light blue, then gold, then lavender. Drawing the EDTA tube before the serum tube (B, D) allows potassium-EDTA carryover that falsely raises potassium and falsely lowers calcium on the chemistry panel.

    Reference Domain 3E; k132 order of draw; CLSI order of draw (PRE02, formerly GP41); additive carryover

  5. A gold serum separator tube has just been collected for a comprehensive metabolic panel. What is the correct handling before centrifugation?

    • ACentrifuge immediately so the serum separates before clotting begins.
    • BInvert about 5 times and let the specimen clot upright for at least 30 minutes at room temperature.Correct
    • CPlace the tube in an ice slurry for 15 minutes, then centrifuge.
    • DShake the tube vigorously to activate the clot activator, then centrifuge.

    Why: Serum separator tubes are inverted about 5 times to mix the clot activator and must clot fully, at least 30 minutes at room temperature, before centrifuging, or latent fibrin will contaminate the serum and clog analyzers. Centrifuging early (A) leaves that fibrin behind. Vigorous shaking (D) hemolyzes the specimen, and routine chemistry serum is not chilled. Serum should be separated from cells within 2 hours of collection.

    Reference Domain 3E; k140 centrifuge and aliquot; CLSI serum processing (clot 30 min, separate from cells within 2 hours); BD Vacutainer SST 5 inversions

  6. A light blue sodium citrate tube collected for a PT/INR was filled only about two-thirds of the way. What should the medical assistant do?

    • ASend the tube; the laboratory will correct the result for the volume drawn.
    • BTransfer blood from a second light blue tube to top off the underfilled tube.
    • CAdd sterile saline to bring the tube up to the fill line.
    • DRecollect the specimen in a completely filled light blue tube.Correct

    Why: The light blue tube depends on a 9:1 blood-to-sodium-citrate ratio; an underfilled tube carries excess anticoagulant and falsely prolongs the PT/INR and aPTT, which could lead a provider to reduce anticoagulant dosing on a false result. The only acceptable correction is recollection in a fully filled tube. Pooling tubes (B) or diluting with saline (C) further corrupts the ratio and is never acceptable.

    Reference Domain 3E; k135 fill level/ratios; CLSI 9:1 blood-to-citrate ratio for coagulation testing

  7. Which action best preserves the quality of a lavender EDTA tube collected for a CBC?

    • AInvert the tube gently 8 to 10 times immediately after collection.Correct
    • BShake the tube briskly for 10 seconds to mix the additive.
    • CLet the tube stand upright for 30 minutes before mixing it.
    • DPlace the tube in the freezer immediately after collection.

    Why: EDTA tubes are inverted gently 8 to 10 times right after collection so the anticoagulant disperses through the sample; a clotted or microclotted EDTA tube is rejected for a CBC because the platelet and cell counts are falsely low. Brisk shaking (B) hemolyzes the specimen, delayed mixing (C) lets microclots form, and freezing (D) ruptures red cells.

    Reference Domain 3E; k135 number of tube inversions; BD Vacutainer inversion guidance (EDTA 8-10 inversions)

  8. A provider orders a blood glucose that will be sent to the reference laboratory later in the day. Which tube preserves the glucose value, and why?

    • ALavender EDTA tube, because EDTA binds glucose in the sample.
    • BLight blue citrate tube, because citrate stabilizes serum glucose.
    • CGray tube with sodium fluoride and potassium oxalate, because fluoride inhibits glycolysis.Correct
    • DGreen heparin tube, because heparin stops red cell metabolism.

    Why: Sodium fluoride is an antiglycolytic agent that slows red cells from consuming glucose, and potassium oxalate is the anticoagulant; without it, glucose falls measurably every hour the specimen sits. This is also why the gray tube is drawn last in the CLSI order, since fluoride and oxalate carryover interferes with electrolytes and enzymes. Heparin (D) prevents clotting but does not stop glycolysis.

    Reference Domain 3E; k136 additives and preservatives; CLSI order of draw (gray fluoride/oxalate last)

  9. A serum specimen arrives at the laboratory with a pink-red tint and is rejected for hemolysis. All of the following practices contribute to hemolysis EXCEPT:

    • AVigorously shaking the tubes to mix the additive
    • BPuncturing the site before the alcohol has dried
    • CDrawing the specimen through a 25 gauge needle
    • DAllowing each evacuated tube to fill under its own vacuumCorrect

    Why: Hemolysis releases intracellular contents and falsely elevates potassium, LDH, and AST, so a hemolyzed specimen must be recollected rather than reported. Shaking, wet alcohol, and forcing cells through an excessively narrow 25 gauge lumen all rupture red cells. Letting the evacuated tube fill under its own vacuum is the correct technique; it is applying extra suction, as with a syringe pulled back too fast, that causes hemolysis.

    Reference Domain 3E; k138 preanalytical considerations; hemolysis causes falsely elevated potassium, LDH, AST

  10. When collecting multiple microcollection containers from a fingerstick, which order should be followed?

    • AEDTA container first, then other additive containers, then the serum containerCorrect
    • BSerum container first, then other additive containers, then EDTA
    • CThe same sequence used for venipuncture
    • DAny sequence, because microcollection containers have no additives

    Why: The capillary order of draw differs from the venipuncture sequence: after any blood gas specimen, EDTA is collected first because capillary blood begins to clot and platelets clump the moment blood contacts the skin, and the CBC is the test most damaged by that clumping. Other additive containers follow and serum containers are collected last. Microcollection containers do carry the same additives as evacuated tubes, so choice D is false.

    Reference Domain 3E; k130 order of draw with microtubes; CLSI GP42 capillary order of draw (blood gas, then EDTA, then other additives, then serum)

  11. A provider orders two sets of blood cultures. The medical assistant scrubs the venipuncture site with 2% chlorhexidine in 70% isopropyl alcohol. Which action best protects the specimen from contamination?

    • APalpate the vein once more with a clean gloved fingertip to confirm its position.
    • BPuncture as soon as the antiseptic is applied so skin flora has no time to return.
    • CLet the antiseptic dry completely, then puncture without touching the site again.Correct
    • DWipe the prepared site with sterile gauze to remove antiseptic residue first.

    Why: Chlorhexidine works through friction plus contact time: scrub the site with friction for a full 30 seconds, then let it dry completely for at least another 30 seconds before puncture. Passive contact without friction does not achieve antisepsis, and puncturing a wet site leaves viable organisms behind. Re-palpating after the prep is one of the most common causes of a contaminated blood culture, and a glove does not make the finger sterile, so if the vein must be relocated the site has to be cleansed again. Wiping the antiseptic off removes its residual activity. The bottle tops are also disinfected with alcohol and allowed to dry before inoculation. A false-positive culture leads to unnecessary antibiotics and admission, so aseptic technique matters more here than on a routine draw.

    Reference NHA CCMA Test Plan Domain 3E Phlebotomy (k130 site preparation; k139 special collections); CLSI M47-Ed2 Principles and Procedures for Blood Cultures (30-second friction scrub, allow at least 30 seconds to dry)

  12. Potassium results from one medical assistant's draws come back high again and again, yet the specimens show no hemolysis and the patients have no symptoms. The assistant routinely coaches patients to pump the fist several times during the draw. What should change?

    • AHave the patient release the fist once blood flows, with no pumping at all.Correct
    • BKeep the tourniquet on for the whole draw so the vein stays adequately filled.
    • CSwitch to a 23 gauge butterfly set so a small vein is easier to enter.
    • DWarm the site for a few minutes before the draw to improve the blood flow.

    Why: Repeated vigorous fist pumping makes the forearm muscles release potassium into the blood trapped below the tourniquet, raising plasma potassium by roughly 1 to 2 mmol/L, enough to push a normal value into the hyperkalemic range even in a specimen with no visible hemolysis. CLSI PRE02 (formerly GP41) permits the patient to close the fist once so the vein becomes prominent, but vigorous hand exercise must be avoided, and the fist is released once blood begins to flow, because holding the clench through the draw also raises potassium. Leaving the tourniquet on for the whole draw adds hemoconcentration and makes the error worse; the standard is under one minute before the vein is entered. A 23 gauge winged set does not fix hemolysis either, because the narrower bore and slower flow increase it, which is why needles finer than 23 gauge are avoided; warming the site addresses hard-to-find veins. Neither one touches potassium released by forearm muscle. A falsely high potassium can trigger a repeat draw or an unnecessary cardiac workup.

    Reference NHA CCMA Test Plan Domain 3E Phlebotomy (k138 preanalytical considerations pertaining to specimen quality); CLSI PRE02 (formerly GP41) Collection of Diagnostic Venous Blood Specimens; Don BR et al., Pseudohyperkalemia Caused by Fist Clenching during Phlebotomy, N Engl J Med 1990;322:1290; Joint EFLM-COLABIOCLI recommendation for venous blood sampling

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