4.1 Equipment & Tube Selection

Why Equipment and Order of Draw Dominate This Domain

Routine Blood Collections is the single largest NHA Certified Phlebotomy Technician (CPT) domain at 28% of scored items (28 of 100). More questions here test tube selection, additive chemistry, and the order of draw than any other concept on the exam. A candidate who can recite the order of draw and match every tube color to its additive and tests has already secured a large share of a passing score.

The governing standard is CLSI GP41 (Clinical and Laboratory Standards Institute, document GP41), the consensus document that defines venipuncture procedure and the order of draw in the United States. NHA items follow this standard, so memorize it exactly as written.

Needles and Gauge

A needle gauge is the diameter of the needle bore. The relationship is inverse: the higher the gauge number, the smaller the bore. This is the single most common equipment trap on the exam.

A needle that is too small (high gauge) forces blood through a narrow bore and can rupture red cells, causing hemolysis. The standard routine choice is a 21-gauge multisample needle.

Three Collection Systems

Three systems can be used to draw venous blood. The exam expects you to know when each is preferred.

Evacuated Tube System (ETS)

The evacuated tube system (ETS) is the routine method. A double-ended multisample needle screws into a tube holder; tubes contain a premeasured vacuum that pulls blood automatically and stops at the correct fill volume. ETS is closed (lower exposure risk) and supports drawing multiple tubes in sequence. It is the default for a routine antecubital draw.

Syringe

A syringe uses manual barrel aspiration and is chosen for fragile, small, or rolling veins where a strong vacuum would collapse the vein. Blood is transferred to tubes afterward using a safety transfer device — never by pushing through the needle, which causes hemolysis and a needlestick risk. When filling tubes from a syringe, you still respect the order of draw.

Winged Infusion (Butterfly) Set

A winged collection set (butterfly) is a short needle with flexible tubing, used for hand, wrist, or small antecubital veins and for many pediatric draws. A 23-gauge butterfly is typical. Key trap: when a light blue (citrate) tube is the first or only tube, draw and discard a clear (non-additive) tube first to prime the air out of the tubing, otherwise the citrate tube underfills and the 9:1 ratio is wrong.

Tube Additives, Colors, and the Tests They Serve

Each stopper color signals a specific additive. The additive determines whether you get serum (no anticoagulant, blood clots first) or plasma/whole blood (anticoagulated). Matching additive to test is heavily tested.

Mechanism rationale you must know:

• EDTA and citrate both chelate calcium, but EDTA binds it irreversibly (good for cell preservation in a CBC), while citrate binds it reversibly in a fixed ratio (required for coagulation testing).
• Heparin does not bind calcium; it accelerates antithrombin to block thrombin, so it is fast and ideal for STAT chemistry but unsuitable for a CBC blood film.
• Sodium fluoride preserves glucose by stopping glycolysis (red cells consuming glucose), preventing a falsely low result.

Citrate fill rule: light blue tubes need a 9:1 blood-to-citrate ratio. An underfilled (short-draw) citrate tube has excess citrate, prolonging clotting times and causing rejection.

CLSI Order of Draw and Why It Exists

The order of draw prevents additive carryover — additive transferred on the needle from one tube into the next, which falsifies results. Always draw in this CLSI GP41 sequence:

1. Blood culture / sterile (yellow SPS bottles) — drawn first to protect sterility before the skin or equipment can contaminate it.
2. Light blue (sodium citrate) — coagulation tube placed early so it is not contaminated by clot activator or other anticoagulants.
3. Red / gold SST (serum, clot activator, gel) — serum tubes; clot activator carryover here would not harm later tubes the way it would harm coagulation.
4. Green (heparin) — heparin carryover into EDTA or citrate would interfere, so heparin follows serum.
5. Lavender / pink (EDTA) — EDTA carried into a chemistry tube falsely lowers calcium and falsely raises potassium, so EDTA comes after chemistry tubes.
6. Gray (fluoride/oxalate) — drawn last because its potent additives cause the worst contamination if carried backward.

Mnemonic to anchor it: Cultures, Light blue, Red, Green, Lavender, Gray ('Stop Light Red Stay Put, Light Green Goes'). The most exam-critical errors are EDTA contaminating a potassium result and citrate or heparin contaminating coagulation.

Inversions: Mix Without Clotting

After filling, tubes with additive are gently inverted (a full 180-degree turn and back is one inversion) to mix the additive. Never shake — shaking causes hemolysis. Plain red has no additive and needs none.

Inadequate EDTA mixing causes clotted CBC specimens, an absolute rejection that requires recollection.