Phlebotomy: Capillary Puncture

Capillary Puncture Overview

  • Capillary puncture involves puncturing the dermal layer of the skin to collect blood. This method is commonly used when only a small amount of blood is needed for testing.
  • Terms include capillary, dermal, and skin puncture. Understanding these terms helps in accurately documenting and communicating the procedure.
  • Capillary specimen and capillary puncture are preferred terms for accuracy.
  • Small sample values are tested using laboratory instruments designed for minimal blood volumes.
  • Procedures align with CLSI standards for finger or heel punctures to ensure consistency and reliability.

Bleeding Time Test

  • A lancet punctures the arm to observe bleeding time, which measures how quickly blood clots.
  • Drops of blood are placed on a numbered card to monitor coagulation progress over time.
  • Prolonged bleeding indicates a coagulation problem that may require further investigation.
  • Finger and heel punctures are the primary methods for obtaining blood samples for this test.

Authorized Puncture Sites

  • Only finger or baby's heel punctures are authorized to minimize risk and discomfort.
  • Puncturing other areas like the leg or butt cheek is prohibited due to potential complications.
  • Adult heels cannot be punctured because the skin is too thick and less vascular.
  • After 12 months or 21 pounds, heel punctures are not allowed as the heel's structure changes.
  • Alternative devices are used for arm punctures in older patients to ensure proper sample collection.

Finger Puncture Lancets

  • Various types and sizes of finger puncture lancets exist, each designed for specific uses and patient types.
  • Different lancets have unique opening mechanisms (e.g., pressing the top or sides) for ease of use and safety.
  • The purple lancet has multiple size options, allowing for adjustable puncture depths.
  • Finger lancets are unsuitable for heel punctures as they don't penetrate deep enough to obtain an adequate sample.

Heel Puncture Lancets

  • Baby lancets are designed specifically for heel punctures to ensure safety and proper depth.
  • Regular-sized lancets (pink and blue) have a maximum depth of 2.02.0 millimeters to avoid injury to the bone.
  • Exceeding this depth is prohibited to prevent complications such as osteomyelitis.
  • White lancets are for premature (PD) babies and penetrate only 11 millimeter to accommodate their thinner skin.
  • Using the white lancet on a regular baby may not yield enough blood, requiring multiple punctures, which should be avoided.
  • Some lancets have narrow plastic tubes for blood collection, simplifying the process.

Microcollection Containers

  • Tubes are color-coded to represent different departments and additives, aiding in quick identification.
  • Lavender tubes contain EDTA (ethylene diamine tetraacetic acid), an anticoagulant used for hematology tests.
  • Tubes have minimum and maximum fill levels, as indicated by lines, to ensure proper additive-to-blood ratio.
  • Insufficient blood may not mix properly with the additive, leading to clotting, while excessive blood can cause clotting due to additive saturation.

Special Containers

  • Amber-colored containers protect samples from light, which can degrade certain analytes.
  • These are used for newborn tests that require light protection; if unavailable, samples can be wrapped in gauze or aluminum foil to protect from light.

Order of Draw

  • The order of draw differs for microcapillary specimens compared to venipuncture due to different clotting dynamics.
  • In microcollection, the lavender tube is drawn first due to its fast clotting time in small samples.
  • This contrasts with venipuncture, where lavender is drawn last to avoid contamination with additives from other tubes.
  • Microcollection involves scooping blood, reducing the risk of contamination from external sources.
  • The microcapillary order of draw is: Blood gas, Lavender, Light Green, other additives, w/ serum last.

Microhematocrit Tubes

  • Micromatic grid tubes (microhematocrit tubes) are used to collect blood for hematocrit determinations. Commonly made of glass.
  • These tubes are fragile and can break easily, posing a risk of injury.

    *Plastic tubes are increasingly common
  • Tubes have a blue line indicating the fill stop point to ensure proper blood volume.
  • Some tubes contain heparin or anticoagulants to prevent clotting, crucial for accurate hematocrit readings.
  • These tubes hold 5050 to 7575 microliters of blood, a small amount used for HCT determinations.
  • One end of the tube is sealed with plastic clay or sealant to prevent leakage during centrifugation.

Capillary Blood Gas (CBG) Collection

  • CBG assesses oxygen levels in the blood, providing insights into respiratory function.
  • Capillary blood is a mixture of arterial and venous blood, offering a representative sample.
  • Tubes usually contain an additive to prevent clotting, ensuring accurate and reliable results.
  • Stirrers or metal fillings are used to mix the blood; alternatives like clean staples and magnets may be used to ensure homogeneity.

Microscope Slides

  • Used for blood films and hematology determinations to examine blood cell morphology.
  • Exhibit ads are used for scientists readings, aiding in accurate analysis.

Warming Devices

  • Warming the puncture site increases blood flow up to seven times, facilitating easier sample collection.
  • Warming is crucial for drawing blood from babies, especially those with jaundice, to improve blood flow and bilirubin measurement accuracy.
  • A warming sticker is applied to the heel before prepping all materials to get them ready, optimizing the process.

Risks of Capillary Puncture

  • Bruising is a common risk, especially in patients with coagulation disorders.
  • Puncture site calcification and scarring, especially in diabetic patients, can occur with repeated punctures.

Composition of Capillary Specimen

  • A mixture of arterial, venous, and capillary blood, reflecting overall blood composition.
  • More closely resembles arterial blood, which is bright red due to high oxygen levels, making it suitable for oxygenation studies.
  • Venous blood is darker due to lower oxygen levels, affecting the overall color of the sample.
  • Excessive squeezing can introduce interstitial and intracellular fluid, affecting test results and diluting the sample.

Reference Values

  • Capillary reference values may differ from venous values due to differences in blood composition.
  • Glucose concentrations are higher in capillary blood, which can impact diabetes management.
  • Bilirubin, calcium, chloride, and sodium levels are lower in capillary blood, requiring careful interpretation.
  • It's important to indicate 'capillary' when submitting capillary blood for testing due to differing values, ensuring accurate laboratory analysis.

Indications for Capillary Puncture

  • Fragile veins make venipuncture difficult and risky.
  • Veins difficult to access due to scarring, obesity, or other factors make capillary puncture a viable alternative.
  • Phlebotomy sites with burns or scars may not be suitable for venipuncture.
  • Patient has clot forming tendencies that contraindicate larger punctures.
  • Patients who are needle phobic may prefer the less invasive capillary puncture.
  • Overweight patients (controversial) may have deep or hard-to-access veins.
  • Inaccessible veins/IVs in both arms limit venipuncture options.
  • POCT procedures, such as testing glucose, often use capillary blood for convenience.
  • Infants and young children: Small blood volume to reduce risk of anemia make capillary puncture preferable.
  • Capillary blood is the preferred specimen for newborn screens, minimizing the amount of blood needed.

Tests Not Collected by Capillary Puncture

  • Coagulation studies requiring plasma specimens typically need larger volumes obtained through venipuncture.
  • Blood cultures require a larger volume to ensure adequate detection of microorganisms.

Order of Draw

  • Blood gas (if applicable) is drawn first to minimize exposure to air.
  • EDTA (e.g., lavender tube) is drawn next for hematology tests.
  • Other additive specimens are collected in the appropriate order to avoid contamination.
  • Serum (last) is collected last to allow for proper clotting.

General Puncture Steps

  • Steps include greeting and identifying the patient, washing hands, and preparing the puncture site for safety and accuracy.
  • Implied consent