7.4 Short Draws, Hemolysis, and Plasma-on-Cells Limits

CMLA II.3.B.8.b–d groups three integrity rules: understand the effect of a short draw; prevent hemolysis and know the effect of a hemolyzed specimen; and know the limit of time that plasma can set on the cells. Order of draw (Section 7.3) keeps additives in the right tube. This section is what happens when the tube is underfilled, the cells are shredded, or the liquid is left sitting on the clot or cell pack. Handling and transport details that AMT lists after the draw—labeling, send-outs, post-care—are Chapter 8. The standard for handling, transport, processing, and storage on AMT CMLA-REF-2025-1 is CLSI PRE04 (1st edition, 2023), not PRE02.

Quick Answer: Short-draw light blue = invalid PT/aPTT. Hemolysis from small needle plus big vacuum, fist pumping, wet alcohol, trauma, or a forced syringe. K, LDH, AST, Mg, and phosphate go up; insulin and some RBC results go down. Follow SOP/PRE04: many routine chemistry specimens need prompt centrifugation so plasma or serum is not left on cells. Glucose falls without fluoride.

Effect of a short draw

A short draw is an underfilled tube. The vacuum stopped early, the vein collapsed, the butterfly line held air, or the collector pulled the tube off too soon. Additive concentration is designed for a full tube. Less blood means more additive per milliliter.

Sodium citrate (light blue) is the highest-yield example. The method assumes a 9:1 blood-to-anticoagulant ratio. Underfill leaves excess citrate, which binds the calcium the assay adds back, so PT, aPTT, and INR prolong artifactually. The result is invalid, not a slightly conservative INR. Recollect; do not top off from another tube.

EDTA (lavender/pink). Excess EDTA shrinks red cells, so hematocrit and mean corpuscular volume can fall and mean corpuscular hemoglobin concentration can rise. Platelets can swell or fragment. A short-draw CBC is not a creative microtainer.

Heparin and serum tubes are less ratio-fragile than citrate, but an almost empty tube may not yield enough plasma or serum, and a skimpy serum tube may not clot or spin correctly.

Blood-culture volume is a recovery problem: too little blood lowers the chance of growing the organism. That is a short-draw effect even though the bottle is not a 9:1 citrate tube.

Expired, cracked, or previously opened evacuated tubes lose vacuum and mimic a hard-stick short draw. Check dating before you blame the vein.

Preventing hemolysis

Hemolysis is rupture of red cells, releasing hemoglobin (pink to cherry-red plasma or serum after centrifugation) and intracellular contents into the liquid.

Prevention is technique: appropriate gauge (often 21–23 gauge for adult antecubital vacuum tubes), dry alcohol, no fist pumping, no hematoma draws, transfer device without plunging, inversion not shaking, tourniquet off by one minute.

Effect of a hemolyzed specimen

Red cells are potassium-rich and enzyme-rich. When they lyse, selected chemistry values move even if the patient is unchanged.

Report or recollect per SOP. Do not average a hemolyzed potassium with a previous result to make it look nicer. Gross hemolysis on a critical potassium is a patient-safety event, not a clerical inconvenience.

Hemoconcentration from a long tourniquet can raise potassium without spectacular pink plasma; hemolysis and hemoconcentration can stack. Both are collector-preventable.

Plasma sitting on the cells: PRE04, not one invented hour

AMT asks you to know the limit of time that plasma can set on the cells. The reference standard is CLSI PRE04 (2023), Handling, transport, processing, and storage of blood specimens for routine laboratory examinations. PRE02 tells you how to collect; PRE04 tells you what happens after.

Do not memorize one universal hour for every analyte as if AMT published a single magic number. Teach this instead: follow SOP and PRE04. Many routine chemistry specimens require prompt centrifugation so plasma or serum is not left sitting on cells. A commonly taught window for many routine tests is within about 2 hours, but special tests differ—some need faster separation or ice, some are stable longer once the liquid is off the cells, gel-barrier tubes help only after a proper spin.

While plasma or serum sits on the cells:

• Glucose decreases as cells continue glycolysis, unless sodium fluoride (gray-top) is present. That is why a delayed red-top or green-top glucose is not equivalent to a properly handled fluoride specimen.
• Potassium can rise as cells leak, especially with delay, cold, or minor hemolysis.
• Other metabolites and enzymes can drift (lactate up, some enzymes unstable).

Gel separators (SST/PST from Chapter 6) slow cell contact after centrifugation lodges the barrier. They do not replace timely spinning. Whole-blood EDTA for a CBC is mixed and not separated, but it still has time and temperature limits in the SOP.

In practice

A light-blue tube stops a third of the way up, the lavender is full but shaken, the chemistry tube is hemolyzed and then left on the clot at the desk until the next shift. You reject the citrate as a short draw, reject the shaken EDTA for hemolysis risk, and know the chemistry potassium, LDH, and AST are not reportable from a hemolyzed, on-the-cells specimen. You recollect, invert by IFU, and get the chemistry tube centrifuged per PRE04/SOP so the liquid is not sitting on the cells. Glucose without fluoride will have fallen while it waited.

Capillary puncture, infant heels, post-care, labeling, transport, processing, send-outs, and legal-evidence criteria move to Chapter 8. This chapter ends when the venous specimen is filled in the right order, mixed, not hemolyzed, and not left on the cells beyond the handling limit.

Exam traps

• Short draw matters only for EDTA (citrate is the classic invalid).
• Hemolysis lowers potassium (it raises K, LDH, AST, Mg, phosphate).
• Plasma may sit on cells overnight if the tube is refrigerated (not a universal truth).
• PRE02, not PRE04, is the handling-time standard.
• Fluoride is optional because glycolysis stops at room temperature.
• Forcing syringe blood is faster and therefore better.

When a stem shows pink plasma, an underfilled blue top, or tubes waiting on the clot, name the ratio, the leaked analytes, and PRE04 prompt separation—not a homemade hour that ignores the SOP.