6.9 Special Collections: GTT, TDM & Blood Bank

6.9 Special Collections: GTT, TDM & Blood Bank

Specialized blood collection procedures require precise technical execution, strict adherence to timing protocols, and enhanced patient preparation beyond standard venipuncture. Because these diagnostic tests evaluate systemic infections, metabolic disorders, pharmaceutical drug levels, and blood transfusion compatibility, improper collection technique directly compromises patient safety and clinical outcome.

Blood Culture Collections & Aseptic Technique

Blood cultures are ordered to detect bacteremia (presence of bacteria in the blood) and septicemia (systemic infection caused by microorganisms multiplying in the blood). Because normal skin flora (such as Staphylococcus epidermidis) can contaminate the specimen and cause false-positive results, strict aseptic technique is mandatory.

Skin Antisepsis Protocol

Standard 70% isopropyl alcohol is insufficient for blood culture antisepsis because it only reduces transient flora. Phlebotomists must follow a two-step or specialized single-step skin cleansing process:

1. Primary Antiseptic: Chlorhexidine gluconate (CHG) is the preferred antiseptic for patients 2 months of age and older. It must be applied using a friction scrub for 30 to 60 seconds and allowed to air-dry completely.
2. Alternative Protocol: If chlorhexidine is contraindicated or unavailable, cleanse the site with 70% isopropyl alcohol for 30 seconds, followed by povidone-iodine or tincture of iodine applied in concentric circles moving outward. The iodine must air-dry for 1 to 2 minutes to eliminate skin pathogens.
3. Critical Rule: Once the site has been cleansed, it must never be repalpated, even with gloved fingers, unless sterile gloves are worn.

Aerobic vs. Anaerobic Order of Draw

Blood culture sets consist of two bottles: an aerobic bottle (contains oxygen for aerobic organisms) and an anaerobic bottle (devoid of oxygen for anaerobic organisms). The bottle filling sequence depends directly on the collection equipment used:

Volume Requirements & Collection Rules

• Adult Volumes: Standard adult sets require 8 to 10 mL of blood per bottle (16 to 20 mL total per set). Underfilling reduces pathogen yield and produces false negatives; overfilling neutralizes the broth's anticoagulant to blood ratio.
• Pediatric Volumes: Pediatric bottles require 1 to 3 mL of blood, adjusted based on the infant or child's body weight.
• Multiple Sets: Blood cultures are typically ordered in sets of two or three, drawn from two separate venipuncture sites 15 to 30 minutes apart. Blood should never be drawn from an existing intravenous (IV) line unless specifically ordered by a physician, as catheter lines harbor high rates of colonizing bacteria.

Glucose Tolerance Testing (GTT)

The Glucose Tolerance Test (GTT)—also referred to as the Oral Glucose Tolerance Test (OGTT)—evaluates carbohydrate metabolism and diagnoses conditions such as Diabetes Mellitus and Gestational Diabetes. GTT measures the body's ability to clear a standardized glucose load from the bloodstream over a timed interval.

Patient Preparation Guidelines

For GTT results to be accurate, the patient must adhere strictly to pre-test instructions:

• Dietary Prep: The patient must eat a balanced diet containing at least 150 grams of carbohydrates per day for 3 consecutive days prior to the test.
• Fasting Requirement: The patient must fast for 8 to 12 hours prior to the baseline blood draw. Water intake is encouraged to maintain hydration, but coffee, tea, gum, smoking, and strenuous exercise are strictly prohibited.

Step-by-Step GTT Protocol

1. Baseline Fasting Draw: Collect a baseline fasting blood specimen (and fasting urine specimen if ordered). Check the fasting blood glucose level immediately via point-of-care testing. If fasting glucose is abnormally high (e.g., >126 mg/dL), notify the physician before proceeding; the test may be canceled for patient safety.
2. Glucola Administration: Administer the standard oral glucose beverage (Glucola), containing 75 grams of glucose for standard adult testing (or 100 grams for 3-hour gestational testing). The patient must drink the entire volume within 5 minutes.
3. Timing Initialization: Start the timer immediately when the patient finishes drinking the glucola.
4. Timed Collections: Collect blood samples at exact scheduled intervals—typically at 30 minutes, 1 hour, 2 hours, and 3 hours post-ingestion. Grey-top tubes containing sodium fluoride (glycolytic inhibitor) and potassium oxalate are standard.
5. Complication Management: If the patient vomits during the test, the test is aborted, the physician is notified, and the procedure is rescheduled for another day.

Therapeutic Drug Monitoring (TDM)

Therapeutic Drug Monitoring (TDM) measures blood concentration levels of medications with narrow therapeutic windows—where the difference between an ineffective dose and a toxic dose is minimal. Common TDM drugs include vancomycin, gentamicin, digoxin, theophylline, and lithium.

Trough vs. Peak Levels

Critical Note: Phlebotomists must coordinate directly with nursing staff to log the exact minute an infusion finishes before timing a peak draw.

Blood Bank Collections & Transfusion Safety

Blood bank specimens (such as Type and Crossmatch, ABO grouping, Rh typing, and antibody screening) carry the highest risk profile in phlebotomy practice. A single labeling or identification error can result in a fatal hemolytic transfusion reaction.

Identification & Labeling Protocol

1. Strict Identification: Blood bank standards require positive verification using two patient identifiers (full legal name and date of birth), matched against the requisition and inpatient wristband.
2. Dedicated Blood Bank Wristband: In addition to the standard hospital ID band, a special blood bank bracelet (Ident-A-Band) containing a unique alphanumeric tracking code is attached to the patient's wrist.
3. Tube Selection: Blood bank collections utilize pink-top tubes containing spray-coated K2EDTA (or lavender-top EDTA tubes), which must be inverted 8 to 10 times immediately after draw.
4. Bedside Labeling: Labels must be hand-written or printed and attached at the patient's bedside immediately after draw. Labels must include the patient's full name, DOB, medical record number, collection date, exact time, phlebotomist's full signature or initials, and matching blood bank band number.