7.3 CLSI GP42 Capillary Order of Draw & Blood Gas Collection

7.3 CLSI GP42 Capillary Order of Draw & Blood Gas Collection

Quick Answer: The CLSI GP42 Capillary Order of Draw is fundamentally different from venous draw: 1) Capillary Blood Gases (CBGs), 2) EDTA Micro-containers (Lavender/Pink), 3) Other Additive Micro-containers (Green/Heparin, Gray/Fluoride), and 4) Serum Micro-containers (Red/Gold SST). EDTA must be drawn first among micro-tubes because platelets aggregate immediately at dermal wound sites; delaying EDTA causes platelet clumping and micro-clots. Neonatal heelsticks are restricted to the medial and lateral plantar borders of the heel with a strict maximum puncture depth of 2.0 mm (0.85–1.5 mm for preemies) to prevent hitting the calcaneus bone and causing osteomyelitis / osteochondritis. Newborn metabolic screening (PKU/Guthrie cards) collected at 24–48 hours requires filling each printed circle with one large continuous drop that saturates through to the back side without touching the paper to the infant's skin or layering drops, followed by 3 to 4 hours of horizontal air drying.

The CLSI GP42 Capillary Order of Draw

A common source of error on certification exams and in clinical practice is confusing the Venous Order of Draw with the Capillary Order of Draw. The sequence of filling micro-collection containers is dictated by cellular physiology and the mechanics of wound hemostasis.

                  CLSI GP42 CAPILLARY ORDER OF DRAW
  ┌────────────────────────────────────────────────────────────────────────┐
  │ 1. CAPILLARY BLOOD GASES (CBGs)                                        │
  │    • Heparinized capillary tubes + flea + magnet.                      │
  │    • Collected FIRST to prevent ambient air exposure.                  │
  ├────────────────────────────────────────────────────────────────────────┤
  │ 2. EDTA MICRO-COLLECTION TUBES (Lavender / Purple / Pink)              │
  │    • Whole blood hematology (CBC, Platelets, Differential).            │
  │    • Collected FIRST among micro-tubes to prevent platelet clumping.   │
  ├────────────────────────────────────────────────────────────────────────┤
  │ 3. OTHER ADDITIVE MICRO-TUBES                                          │
  │    • Green / Light Green (Lithium/Sodium Heparin - Plasma Chemistries) │
  │    • Gray (Sodium Fluoride / Potassium Oxalate - Glucose)              │
  ├────────────────────────────────────────────────────────────────────────┤
  │ 4. SERUM MICRO-COLLECTION TUBES (Red / Gold / Amber SST)               │
  │    • Chemistry, serology, immunohematology.                            │
  │    • Collected LAST because clotting is desired for serum separation.  │
  └────────────────────────────────────────────────────────────────────────┘

Why Capillary Order Differs from Venous Order

In routine venipuncture, blood enters a closed, sealed vacuum tube system directly from the vein lumen without contacting extravascular tissue. In contrast, a dermal puncture creates an open wound where blood passes across severed collagen fibers and exposed tissue cells:

1. Immediate Platelet Activation: The moment skin capillaries are incised, the primary hemostatic plug begins forming. Platelets adhere to exposed subendothelial collagen and release ADP and thromboxane $A_2$, causing rapid platelet aggregation at the puncture site.
2. Preventing Hematology Invalidation: If micro-collection tubes containing EDTA (Lavender) are drawn after other tubes, the later drops of blood will contain clumped platelets and microscopic fibrin micro-clots. This produces falsely decreased platelet counts (pseudothrombocytopenia) and distorts automated CBC leukocyte differentials.
3. Why Serum is Drawn Last: Serum micro-containers (Red/Gold) rely on natural blood clotting to yield serum after centrifugation. Because clotting is the intended outcome, collecting serum tubes last has zero adverse impact on test validity.

Comparison: Venous vs. Capillary Order of Draw

Capillary Blood Gas (CBG) Collection Mechanics

Capillary Blood Gas (CBG) analysis is frequently ordered on neonates and pediatric intensive care patients to evaluate blood pH, partial pressure of carbon dioxide ($pCO_2$), and partial pressure of oxygen ($pO_2$) when arterial puncture or arterial line access is unavailable.

                    CAPPILLARY BLOOD GAS (CBG) ASSEMBLY
  ┌────────────────────────────────────────────────────────────────────────┐
  │   [Cap] ┌───────────────────────────[Flea]───┐ [Cap]                   │
  │   ===== │  Heparinized Capillary Tube (Glass) │ =====                   │
  │         └────────────────────────────────────┘                         │
  │                            ▲                                           │
  │                            │ (Exterior Magnet slides flea back/forth)  │
  │                         [MAGNET]                                       │
  └────────────────────────────────────────────────────────────────────────┘

CBG Equipment & Procedural Steps

• Specialized Equipment:
  • Heparinized Capillary Tube: Narrow-bore glass or plastic tube lined with balanced lithium or sodium heparin anticoagulant.
  • Metal Flea: A small, surgical stainless steel metal bar/pin inserted into the tube.
  • Mixing Magnet: An external handheld magnet used to guide the internal flea.
  • Rubber/Plastic Sealing Caps: Airtight caps for both ends of the capillary tube.
• Technique:
  1. Pre-Warm Site: Warm the heel or finger for 5 full minutes at 40°C–42°C. Thorough warming is mandatory to arterialize the capillary bed so that $pCO_2$ and pH closely correlate with arterial values.
  2. Anaerobic Collection: Perform puncture, wipe first drop, and hold the capillary tube horizontally directly against the emerging blood drop. Fill the tube completely from end to end without air bubbles.
  3. Insert Flea & Seal: Insert the metal flea into the tube, immediately seal both ends with airtight caps to prevent room air exposure, and slide the external magnet back and forth smoothly along the tube exterior to mix the heparin without agitating the specimen.
  4. Transport: Transport immediately to the laboratory on an ice-water slurry for analysis within 15 minutes.