Capillary Order of Draw and Device Types

Why Capillary Collection Has Its Own Order of Draw

When you puncture a fingertip or heel, tissue fluid mixes with blood almost immediately. That tissue fluid contains thromboplastin, a substance that triggers clotting. Because of this, the capillary order of draw is not the same as the venous order of draw. Understanding why — and following the sequence without shortcuts — protects specimen quality and, ultimately, the patient whose results depend on those specimens.

The Capillary Order of Draw

The Clinical and Laboratory Standards Institute (CLSI) guideline GP42 establishes the recommended capillary order of draw. Collect tubes and devices in this sequence:

1. Blood gases / capillary blood gas tubes (if ordered) — collected first because speed and a closed system are critical; exposure to air alters results immediately.

2. EDTA tubes (lavender/purple cap microtainers) — collected before other additive tubes to minimize platelet clumping caused by tissue thromboplastin. Accurate platelet counts and complete blood counts depend on this.

3. Other additive tubes — heparin (green cap), serum separator or plain (red/gold cap), and similar chemistry or coagulation tubes, in the order your facility protocol specifies.

4. Serum or non-additive tubes — collected last in capillary collection, the reverse of what many students expect from venous practice.

> Key difference from venous collection: In venous draws, EDTA (purple top) is drawn after coagulation tubes. In capillary collection, EDTA is drawn before chemistry and serum tubes. The reason: tissue thromboplastin released at the puncture site can activate clotting factors and reduce platelet counts if the EDTA tube is not captured early, before the blood begins to change.

Collection Devices: Microtainers

Microtainers are small plastic tubes, typically holding 250–500 µL, designed for capillary specimens. They are available with the same additive options as full-size vacutainer tubes — EDTA, lithium heparin, serum separator gel, and non-additive — identified by color-coded caps.

Correct fill technique:

• Hold the microtainer at a slight downward angle so gravity assists flow.
• Touch the tip of the tube (or its collection scoop) to the drop of blood forming at the puncture site — do not press the tube against the skin and scoop, which introduces tissue fluid and air bubbles.
• Allow the drop to flow into the tube by capillary action and gravity; reposition the finger if the drop stops forming.
• Fill to the indicated fill line; underfilling an additive tube produces an incorrect blood-to-additive ratio, causing erroneous results.
• After filling, cap the tube immediately and gently invert it the number of times specified by the manufacturer (commonly 8–10 times for EDTA, fewer for serum tubes). Vigorous shaking causes hemolysis.

Rejection criteria: A microtainer is rejected by the laboratory if it is underfilled, hemolyzed, clotted (when an anticoagulant was required), mislabeled, or collected in the wrong tube type for the test ordered.

Collection Devices: Capillary Tubes

Capillary tubes (also called microhematocrit tubes) are narrow glass or plastic tubes used primarily to collect blood for hematocrit determinations. They fill by capillary action alone — simply touch one end to the blood drop and the tube draws blood in.

• Fill to approximately three-quarters of the tube length, leaving room to seal one end with clay or a cap.
• Avoid air bubbles; a bubble in a hematocrit tube causes a false reading.
• Some capillary tubes contain heparin (indicated by a red or green band); plain tubes have a blue band. Use the anticoagulated type for whole-blood tests.
• Handle carefully — glass capillary tubes can break and create a sharps hazard. Many facilities now use plastic tubes to reduce injury risk.

Collection Devices: Filter Paper Cards (Newborn Screening Cards)

Filter paper cards, often called Guthrie cards or newborn screening cards, are used to collect blood for newborn metabolic screening. Each circle on the card must be filled completely and evenly with a single application of blood.

• Do not layer drops — applying multiple small drops on the same circle creates a layered (multilayer) specimen that produces inaccurate results.
• Do not touch the filter paper with your fingers; skin oils contaminate the specimen.
• Allow each circle to fill from a single large drop; the blood should soak through and appear uniform on both sides of the card.
• Allow the card to air-dry horizontally for the time specified by the kit instructions before packaging. Do not stack wet cards or apply heat.
• Label the card with the patient's information, not the envelope, so identification travels with the specimen.

Preventing Common Errors

| Error | Consequence | Prevention |

|---|---|---|

| Scooping or scraping tube against skin | Air bubbles, tissue fluid contamination | Touch drop to tube tip only |

| Vigorous shaking of additive tube | Hemolysis | Gentle inversion only |

| Underfilling an additive tube | Wrong additive-to-blood ratio, erroneous result | Fill to the marked line |

| Drawing EDTA after chemistry in capillary collection | Platelet clumping, altered CBC results | Follow capillary order of draw |

| Multiple drops on one filter paper circle | Layered specimen, inaccurate newborn screen | One drop per circle |

Safety First

Always follow standard precautions. Dispose of lancets immediately in an approved sharps container. If using glass capillary tubes, inspect for cracks before use and handle with care. Label every tube before leaving the patient's side. If a specimen is in doubt — clotted, hemolyzed, or improperly collected — recollect rather than report a potentially wrong result. Patient safety depends on specimen integrity.