Comprehensive Notes on Medical Laboratory Science: Dermal Puncture, Special Collections, and Arterial Procedures

Dermal Puncture: Equipment and Blood Composition

Module 1 focuses on the principles and practice of dermal punctures, a vital skill in clinical laboratory science. Dermal puncture devices, specifically lancets, are critical for obtaining high-quality microsamples. The BD Microtainer Contact-Activated Lancet (Becton Dickinson) is engineered for safety and precision, activating only when the blade or needle is positioned and pressed firmly against the skin. These lancets are color-coded to identify different puncture depths. THE BD QUIKHEEL LANCETS are specialized, color-coded heel stick lancets designed specifically for premature infants, newborns, and babies. Transitioning to older patients, the TENDERFOOT AND TENDERLETT DEVICES are designed for heel punctures (Tenderfoot) and finger punctures (Tenderlett). Models range from the Tenderfoot for preemies to the Tenderlett for toddlers, juniors, and adults.

Understanding the composition of capillary blood is essential for interpreting test results. Capillary blood is a mixture of arterial and venous blood, primarily dominated by arterial blood due to arterial pressure. Additionally, it may contain small amounts of interstitial fluid (fluid between cells) and intracellular fluid (fluid within cells). Because of this composition, results may differ slightly from venous samples.

Microcollection Containers and Puncture Depth

Microsample containers are used to collect and store the volumes of blood obtained via dermal puncture. CAPILLARY TUBES, also known as micro hematocrit tubes, are small tubes designed to collect approximately $50$ to $75\,\mu L$ of blood, primarily for micro hematocrit testing. These tubes are color-coded: a red band indicates a heparinized tube (to prevent clotting), while a blue band signifies a plain tube (no additive). These are designed to fit perfectly into a hematocrit centrifuge and reader. For larger volumes, the MICROTAINER is used. It provides a collection capacity of approximately $600\,\mu L$ and is made of plastic to eliminate the danger of broken glass.

Precise puncture depth is required to ensure sufficient blood flow while avoiding bone injury. For a NEWBORN, the depth should be between $0.35$ and $1.6\,mm$ below the skin. For ADULTS, the standard depth is $3.0\,mm$ below the skin. Proper site selection and device choice are dictated by these physiological parameters.

Site Selection for Dermal Punctures

Site selection differs based on the patient's age and physical development. For HEEL PUNCTURE SITES, which are used for infants younger than $1$ year of age, the puncture must be performed on the medial or lateral plantar surfaces of the heel. Punctures must NOT be performed in other areas of the foot, particularly the arch, to avoid damaging nerves, tendons, and cartilage. For FINGER PUNCTURE SITES, used for adults and children over $1$ year of age, the preferred locations are the fleshy areas near the center of the third (middle) and fourth (ring) fingers on the palmar side of the non-dominant hand. The tips and sides of the finger should be avoided because they contain only about half the tissue mass of the central area, increasing the risk of bone injury.

Dermal Puncture Procedure and Quality Control

The protocol for dermal puncture involves several critical steps to ensure patient safety and sample integrity. The procedure begins with the requisition form, wearing proper personal protective equipment (PPE) like gowns and gloves, and extensive hand washing. Patient identification is paramount, involving red-lining the requisition form, verbal identification, and checking the ID band. For pediatric outpatients, verbal identification must be obtained from the parents. During the procedure, the patient must be positioned correctly: seated or lying down with the hand supported on a firm surface, palm up, and fingers pointed downward for finger sticks. For heel sticks, infants should lie on their back with the heel in a downward position.

CLEANSING THE SITE is performed with $70\%$ isopropyl alcohol using a circular motion. The alcohol must be allowed to air-dry for maximum antiseptic action and to prevent interference with testing. FAILURE TO ALLOW THE ALCOHOL TO DRY results in: 1. A stinging sensation for the patient; 2. Sample contamination; 3. Hemolysis of Red Blood Cells (RBCs); and 4. Prevention of a rounded blood drop formation (blood will mix with alcohol and run down the finger). Note that povidone-iodine is NOT recommended for dermal punctures, as it can contaminate the sample and artificially elevate results for BILIRUBIN, PHOSPHORUS, URIC ACID, and POTASSIUM.

WARMING THE SITE is another crucial step. Warming dilates the blood vessels and increases arterial blood flow. This is achieved by moistening a towel with warm water (at approximately $42\,^{\circ}C$) or using a commercial heel warmer for $3$ to $5$ minutes. The site should never be warmed for longer than $10$ minutes, as this can alter test results.

Step-by-Step Puncture and Collection Order

When PERFORMING THE PUNCTURE, the heel or finger must be well-supported and held firmly without squeezing. Massaging the area before the puncture can increase blood flow. For a finger puncture, the finger is held between the non-dominant thumb and index finger. For a heel puncture, the heel is held with the index finger over the heel and the thumb below. The puncture device must be placed firmly on the site without indenting the skin. The blade should be aligned to cut across (perpendicular to) the grooves of the fingerprint or heel print to allow blood to form a drop.

Crucially, the first drop of blood must be wiped away with clean gauze to remove tissue fluid contamination. Blood should flow freely; "milking" the site (excessive squeezing) is forbidden as it introduces tissue fluid and causes hemolysis. The ORDER OF COLLECTION for dermal punctures is specific: 1. Capillary blood gases; 2. Blood smear; 3. EDTA tubes (lavender); 4. Other anti-coagulated tubes; and 5. Serum tubes (red/yellow). Microcollection tubes should be slanted down during collection to allow blood to run through the capillary scoop. Collection should be completed within $2$ minutes to prevent micro-clot formation.

Specialized Specimen Collection: Blood Banking and Cultures

Module 2 covers specialized procedures that require additional preparation. BLOOD BANK SPECIMENS are critical for determining blood products for transfusion and include tests like blood typing, cross-matching, and Coombs testing. Labelling must be exceptionally strict, requiring the patient's full name (First with M.I. or Family name with first name), Hospital ID, Social Security number (for outpatients), date of birth, date/time of collection, and phlebotomist initials. BLOOD CULTURE SPECIMENS are collected to detect pathogens, bacteremia, or septicemia in patients with fever of unknown origin. This requires $2$ to $4$ blood culture sets drawn $30$ to $60$ minutes apart. Antiseptics like Betadine, Chloroprep, or $70\%$ ethyl alcohol are used to ensure the site is sterile. For BACTEC or BACT/ALERT systems, a friction rub of $30$ to $60$ seconds is required, followed by disinfection with Chlorhexidine or Povidone-iodine.

Coagulation and Tolerance Testing

COAGULATION SPECIMENS are used to assess clotting functions through tests like Prothrombin Time (PT), Activated Partial Thromboplastin Time (aPTT), and Thrombin Time (TT). These utilize the light blue top tube containing Sodium Citrate. The preferred concentration is $3.2\%$ because it is stable and matches plasma osmolality. The anticoagulant-to-blood ratio is critical at $1:9$; CLSI guidelines require a fill level within $\pm 10\%$ of the mark.

Metabolic tests include the 2-HOUR POSTPRANDIAL GLUCOSE (2-Hour PP), where the patient consumes a meal with $100\,g$ of glucose after fasting for $10$ hours, and blood is collected $2$ hours later. The ORAL GLUCOSE TOLERANCE TEST (OGTT) is diagnostic for diabetes and requires an $8$-hour fast followed by a $75\,g$ glucose load. The ORAL GLUCOSE CHALLENGE (OGC) is a screening test for gestational diabetes involving a $50\,g$ load with a $1$-hour measurement, no fasting required. The LACTOSE TOLERANCE TEST follows similar procedures but substitutes lactose for glucose to detect mucosal lactase deficiency.

Forensic, Toxicology, and Therapeutic Monitoring

THERAPEUTIC DRUG MONITORING (TDM) establishes appropriate dosages by measuring "peak" levels (drug toxicity screening) and "trough" levels (ensuring therapeutic range). This is specifically important for aminoglycoside drugs like Amikacin, Gentamicin, and Tobramycin due to their short half-lives. THERAPEUTIC PHLEBOTOMY involves removing approximately $500\,mL$ of blood to treat polycythemia (overproduction of RBCs) or hemochromatosis (excess iron).

TOXICOLOGY AND FORENSIC SPECIMENS (including blood alcohol/ETOH and drug screening) require a strict chain of custody. This documentation tracks the specimen from collection to results. For drug screening, a licensed proctor must be present, and the collection site (toilet) must have no water to prevent tampering. TRACE ELEMENTS (Aluminum, Arsenic, Copper, Lead, Iron, Zinc) must be collected in special, element-free royal blue tubes to prevent contamination.

Handling, Processing, and Point-of-Care Testing

Module 3 details the handling of specimens. Routine tubes require $3$ to $10$ gentle inversions for proper additive distribution. Specimens should be transported with the stopper up to minimize agitation and facilitate clotting. SPECIAL HANDLING includes: 1. BODY TEMPERATURE ($37\,^{\circ}C$) for Cold agglutinins or Cryoglobulins; 2. CHILLED (crushed ice slurry) for Ammonia, Lactic acid, ACTH, and Gastrin; 3. LIGHT-SENSITIVE (foil-wrapped) for Bilirubin, Vitamin B12, and Folate. Routine blood specimens should reach the lab within $45$ minutes and be centrifuged within $1$ hour. Exceptions include STAT orders and EDTA CBCs (stable for $6$ hours at room temperature, $24$ hours refrigerated).

POINT-OF-CARE-TESTING (POCT) refers to any analytical test performed outside the centralized lab, such as at the patient's bedside. While convenient for shortening Turnaround Time (TAT), factors can affect reliability. Common POCT includes Glucose (HGT), Pregnancy tests, Cardiac Troponin T and I, and Urinalysis.

Arterial Puncture and Blood Gas Analysis

Module 4 focuses on ARTERIAL PUNCTURE for Arterial Blood Gas (ABG) analysis. This invasive procedure evaluates respiratory function, oxygenation, and acid-base balance. Core analytes include: pH ($7.35$ to $7.45$), $pO_2$ ($80$ to $100\,mmHg$), $pCO_2$ ($35$ to $45\,mmHg$), and $HCO_3$ ($22$ to $26\,mEq/L$).

The preferred site is the RADIAL ARTERY due to good collateral circulation and ease of palpation. Other sites include the BRACHIAL ARTERY (deeper, risk of nerve damage) and FEMORAL ARTERY (large, but high infection risk). Preparation requires the patient to be in a "steady state" for $20$ to $30$ minutes. The MODIFIED ALLEN TEST is mandatory before a radial puncture to ensure collateral circulation: the radial and ulnar arteries are compressed, and the ulnar is released; a positive result (hand flushes pink) indicates safe proceedance.

During the procedure, the needle is inserted at a $30$ to $45$-degree angle ($10$-degree for local anesthesia/wheal). Potential hazards include arteriospasm, hematoma, and thrombus formation. ABG specimens should be rejected if they contain air bubbles, have clotted, have hemolyzed, or if the processing time exceeds $10$ minutes from collection.