Green Top Tube

The additive is heparin, either lithium heparin (the most common, used for general chemistry) or sodium heparin (used when sodium values do not matter to the test, mainly ammonia and a few specialty assays).

Heparin works by activating antithrombin III, which then inhibits thrombin and factor Xa. No thrombin, no fibrin, no clot. The blood stays liquid and can be centrifuged in minutes rather than waiting for clot formation.

A PST (plasma separator tube) is a green top with a separator gel, the heparin equivalent of an SST. PSTs are increasingly common for STAT chemistry where the lab needs a barrier-protected plasma.

STAT chemistry: electrolytes, glucose, BUN, creatinine, calcium. The 15-minute turnaround vs 60+ for serum is the reason ERs use green tubes for the rapid panels.

Ammonia testing requires sodium heparin specifically. Lithium heparin contains lithium, which would interfere with ammonia analyzers in some protocols. Always check which heparin variant your facility stocks.

Lactate, certain drug levels, chromosomal analysis (cytogenetics) for karyotyping. Some specialty tests want plasma rather than serum.

Draw 4 to 10 mL. Green tubes come in multiple sizes; the chemistry analyzer typically wants 3 mL of plasma minimum.

Invert 8 to 10 times immediately. Heparin is dry-coated on the tube wall in most modern Vacutainers and needs full mixing. Skip inversions and you get partial clotting.

Heparin tubes cannot be used for coagulation testing. Heparin is itself an anticoagulant. PT and aPTT on a heparin tube are meaningless. If you need both chemistry and coag, draw the citrate first.