3.4 Capillary Equipment & Lancet Depth Selection

3.4 Capillary Equipment & Lancet Depth Selection

Quick Answer: DTP task 2A.5 requires selecting capillary puncture equipment for the test ordered and the type of patient. The controlling variables are lancet depth (matched to age and site, never exceeding 2.0 mm for infant heelsticks), container type (micro-collection tubes, capillary tubes, or filter paper), and warming (never above 42 degrees Celsius).

Lancets and Incision Devices

A lancet is a single-use, sterile device that produces a controlled puncture or incision. Modern devices are retractable and self-disabling: the blade or needle withdraws permanently after firing, so the device cannot be reused even accidentally.

Depth Is the Safety-Critical Variable

The consequence of excessive depth is not a deeper drop of blood — it is bone. In an infant heel, the calcaneus lies as little as 2.0 mm beneath the plantar skin surface, and puncturing it can cause osteochondritis or osteomyelitis, a bone infection with permanent consequences.

[!WARNING] Never use a surgical blade or an uncontrolled-depth device for a heelstick. Depth-limited lancets exist precisely because the operator cannot reliably control depth by feel. Likewise, never re-puncture the same site to extract more blood — obtain a new site with a new lancet.

Incision Width Matters Too

For newborn screening, a device producing a slightly wider, shallow incision yields better free-flowing blood volume than a narrow deep puncture. This is why heelstick devices are specified by both depth and blade length. Excess squeezing to compensate for an inadequate device is exactly what contaminates the specimen with interstitial fluid.

Collection Containers

Micro-Collection Containers

These small plastic containers — commonly known by the brand name Microtainer — collect blood by drop, running along an angled collection scoop into the reservoir. They carry the same cap color coding as evacuated tubes:

The critical difference from evacuated tubes is that there is no vacuum. Blood enters by gravity and capillary action, so:

• Fill to the marked line — under-filling an EDTA micro-container distorts hematology indices exactly as it does in a full-size tube.
• Invert as directed immediately after filling, typically 8 to 10 times for additive containers.
• Do not scoop blood off the skin surface. Scraping the container edge across the skin hemolyzes the sample and drags in epithelial cells and antiseptic residue.

Capillary Tubes

Thin glass or plastic tubes used for microhematocrit and capillary blood gases. They are filled by capillary action from a free-flowing drop and then sealed with clay or a plug.

• Heparinized capillary tubes (green band) are used for microhematocrit and blood gases.
• Plain capillary tubes (blue band) are used when an anticoagulant would interfere.
• Plastic or Mylar-wrapped tubes are preferred over bare glass because of breakage and injury risk.
• Capillary blood gas tubes must be filled without air bubbles and mixed — often with a small magnetic stirrer bar and external magnet, or by rolling — then sealed at both ends immediately.

Filter Paper Collection Cards

Newborn metabolic screening uses a printed filter paper card rather than a tube. The paper itself is the analytical medium and is manufactured to a defined absorbency, which is why the circles must be filled from one side only, in a single application, and why the card cannot be touched, heated, or stacked while drying.

Warming Devices

Warming the site is the most effective way to improve capillary flow, increasing blood flow to the area substantially by arterializing the sample.

• Use a commercial heel warmer or a cloth moistened with warm water.
• Do not exceed 42 degrees Celsius (about 108 degrees Fahrenheit). Infant skin burns readily, and a thermal injury on a newborn heel is a reportable event.
• Warm for approximately 3 to 5 minutes.
• Never use a microwave to heat a compress — heating is uneven and produces scald injuries.

Ancillary Equipment

• Gauze, not cotton balls, for wiping the first drop — cotton fibers adhere to the puncture site and disturb the platelet plug on removal.
• 70% isopropyl alcohol pads only; povidone-iodine is contraindicated for dermal puncture because residual iodine falsely elevates potassium, phosphorus, and uric acid.
• No adhesive bandages for infants under 2 years, because they present a choking and skin-injury hazard on fragile skin.