Gray Top Tube

Two additives. Sodium fluoride is a glycolysis inhibitor. It stops red blood cells from continuing to consume glucose after the draw. This matters because untreated blood loses glucose at roughly 7 percent per hour at room temperature, and the result no longer reflects the patient.

Potassium oxalate is the anticoagulant. It binds calcium, like EDTA, but is less commonly used for downstream tests because it damages cell membranes (which makes it useless for cell counts).

Some gray tubes use sodium iodoacetate instead of fluoride for the same glycolysis-inhibitor function. The replacement is rare in the US but standard in some other countries. The cap color is the same; check the label.

Fasting glucose, especially when transit delays are expected. The fluoride buys time. A gray tube glucose drawn at 7 a.m. and run at 2 p.m. still reflects the 7 a.m. patient.

Glucose tolerance tests (GTT) where multiple draws are taken over hours and the lab needs each draw to reflect the time it was collected, not when it was processed.

Lactate testing, where rapid in-tube glycolysis would falsely elevate the lactate result. Fluoride stops the metabolism and preserves the patient lactate.

Blood alcohol (BAC) for legal and clinical purposes. Sodium fluoride prevents bacterial fermentation of any sugars in the sample, which would otherwise produce ethanol artifactually. This is why forensic BAC always uses gray.

Draw 2 to 10 mL. Gray tubes come in multiple sizes; lab volume requirements vary.

Invert 8 to 10 times immediately. Both additives need to mix throughout the blood quickly. Glycolysis starts within seconds; the longer you wait to invert, the more glucose you lose before the fluoride takes effect.

Always position 6 (last) in the order of draw. No exceptions. If gray is the only tube being drawn, draw it normally; the order rule applies when multiple tubes are collected.