Lavender Top Tube / Purple
EDTA (K2 or K3)
The lavender (purple) top is the most-drawn tube in any clinical lab. EDTA chelates calcium, blood cannot clot, and cell morphology stays intact long enough for a CBC to actually mean something. Get this tube right and you get the most common test on the menu right.
The additive.
The additive is EDTA, dipotassium (K2) or tripotassium (K3). It binds calcium ions, which the coagulation cascade depends on at multiple steps. No calcium, no clot. The cells stay suspended and individually visible under a microscope.
EDTA is dry-coated on the inside of the tube wall in most modern Vacutainers. That is why the inversion count matters: 8 to 10 gentle end-over-end inversions to dissolve the EDTA into the entire blood volume. Skip the inversions and you can get partial clotting at the bottom of the tube. The lab will reject it and you will redraw.
Pink top tubes also contain EDTA but are reserved for blood bank specimens because of their special labeling requirements. Functionally the additive is the same; the tube is a workflow control.
What the lavender top is used for.
The flagship test is the complete blood count (CBC). EDTA preserves cell membranes and platelet morphology better than any other anticoagulant. Hemoglobin, hematocrit, white cell differential, and platelet count all come from a lavender top.
ESR (erythrocyte sedimentation rate) and HbA1c (glycated hemoglobin for diabetes monitoring) also use EDTA. So does ABO/Rh blood typing in many institutions, although blood bank work often prefers pink tops for crossmatch.
EDTA is a poor choice for chemistry tests. It chelates calcium and binds magnesium, which means a chemistry result on a contaminated tube reads as critically low calcium and elevated potassium. The lab will see this on the panel and reject. The order of draw exists in part to keep EDTA away from chemistry tubes.
How to draw it correctly.
Draw 4 to 6 mL. Most lavender tubes are designed to fill to a fixed line; underfill changes the EDTA-to-blood ratio and shifts cell morphology. Lab analyzers detect underfilled lavenders and reject them.
Invert 8 to 10 times immediately after the draw. Gentle inversions, not shaking. Hemolysis from rough handling is one of the top reasons CBC results get flagged.
CLSI position 5. Lavender draws after the green heparin tubes and before the gray fluoride tube. Drawing lavender earlier risks EDTA carryover into chemistry tubes. Drawing it after gray risks fluoride carryover into the CBC analyzer.
Three mistakes that cost you a recollect.
- 1
Underfilling
The EDTA-to-blood ratio shifts. Cell morphology changes. The CBC differential gets weird and the lab rejects.
- 2
Skipping inversions
Partial clotting at the bottom of the tube. Sometimes the analyzer flags it; sometimes it just under-counts platelets and you do not catch it until the redraw.
- 3
Drawing before chemistry
EDTA carryover. Calcium plummets, potassium spikes, the patient looks like they are crashing. Always draw lavender after the chemistry tubes.
Lavender in the order of draw.
Lavender draws fifth in the CLSI sequence: blood culture, light blue, red and gold (serum group), green, lavender, gray. Drawing it earlier than position five means risking EDTA carryover into a tube that will be tested for calcium, potassium, or any chemistry the EDTA can interfere with.
See the full CLSI order-of-draw page →Common questions about the lavender top.
What is the lavender top tube used for?
How many times do I invert a lavender tube?
Lavender vs purple top tube, same thing?
Why does EDTA carryover matter?
Pink vs lavender, what is the difference?
Six more tubes you will see on the same tray.
- Light Blue Light BlueCLSI 2 · Sodium citrate 3.2%
- Red RedCLSI 3 · None (glass) or clot activator (plastic)
- Gold Gold/ SSTCLSI 3 · Clot activator + thixotropic gel
- Green GreenCLSI 4 · Lithium or sodium heparin (PST has gel)
- Gray GrayCLSI 6 · Sodium fluoride + potassium oxalate
- Royal Blue Royal BlueCLSI varies · Trace-element-free (EDTA or no additive)