3.3 Special Blood Collections (Timed, Fasting, GTT, TDM)

Special Blood Collections (Timed, Fasting, GTT, TDM)

Routine venipuncture procedures follow standardized collection steps, but specialized diagnostic pathways require precise patient preparation, rigorous timing protocols, complex handling conditions, or strict legal chain-of-custody documentation. As an ASCP-certified phlebotomy technician, understanding the physiological rationales behind these special blood draws is paramount to ensuring accurate diagnostic outcomes and maintaining specimen validity.

---

Fasting Specimen Collection Protocols

Fasting specimen orders require that the patient refrain from eating or drinking anything except plain water for a designated duration prior to blood collection.

Physiological Basis of Fasting

Ingestion of food triggers complex digestive and metabolic cascades. Consuming carbohydrates elevates blood glucose levels, while dietary fats cause lipemia—an accumulation of chylomicrons and triglycerides that turns plasma milky white and interferes with optical laboratory assays.

• Standard Fasting Duration: 8 to 12 hours NPO (nil per os, or nothing by mouth). Fasting should not exceed 16 hours, as prolonged starvation induces ketosis and alters metabolic baselines.
• Permissible Intake: Patients may drink plain water only. Water consumption is strongly encouraged because dehydration decreases vascular volume, causing veins to collapse and elevating hemoconcentration.
• Prohibited Substances: Coffee (even black without sugar or cream, as caffeine stimulates glycogenolysis), tea, chewing gum (including sugar-free gum, which triggers enzymatic secretions), tobacco products (nicotine alters glucose and cortisol levels), and prescription medications unless specifically authorized by the ordering physician.

Common Tests Requiring Fasting

1. Lipid Panels: Total cholesterol, triglycerides, HDL, LDL, and VLDL.
2. Fasting Blood Glucose (FBG): Baseline screening for diabetes mellitus.
3. Serum Insulin & Gastrin Levels: Endocrine evaluations sensitive to nutrient intake.

---

Oral Glucose Tolerance Testing (GTT)

Glucose Tolerance Testing evaluates the body's physiological ability to metabolize a standardized overload of glucose over a specific timeframe. It is the definitive diagnostic standard for diagnosing Diabetes Mellitus and Gestational Diabetes Mellitus (GDM) in pregnant women.

Step-by-Step GTT Protocol

1. Pre-Test Dietary Preparation: For 3 days prior to the test, the patient must consume a balanced diet containing at least 150 grams of carbohydrates per day, followed by an 8- to 12-hour overnight fast.
2. Baseline Fasting Draw: Upon arrival, the phlebotomist draws a baseline fasting blood specimen (and collects a fasting urine sample if ordered).
3. Fasting Glucose Safety Verification: The phlebotomist must verify the fasting glucose level immediately (via POCT analyzer or rapid laboratory spin). If the baseline fasting glucose exceeds 126 mg/dL (or facility threshold), the phlebotomist must stop the procedure immediately and notify the physician. Administering a high sugar load to a severely hyperglycemic patient can precipitate hyperosmolar nonketotic coma.
4. Glucose Drink Administration: If the fasting baseline is safe, the patient is given a standardized oral glucose solution (75 grams of glucose for non-pregnant adults; 50g or 100g for specific gestational screens). The patient must drink the entire beverage within 5 minutes.
5. Timed Specimen Collection: Timing begins the exact moment the patient finishes drinking the beverage. Blood draws are performed at precise timed intervals: typically at 1 hour and 2 hours post-ingestion.

Strict Patient Rules During GTT

• The patient must remain seated in the waiting area resting quietly throughout the entire test.
• No eating, drinking (except small sips of water), smoking, or physical exertion is allowed. Exercise increases glucose utilization by skeletal muscle, causing falsely low glucose readings.
• Vomiting Protocol: If the patient vomits up the glucose drink at any point during the test, the test is invalidated and aborted immediately. The phlebotomist must notify the physician, document the event, and reschedule the test for a later date.

---

Therapeutic Drug Monitoring (TDM)

Therapeutic Drug Monitoring measures blood concentrations of pharmaceutical agents that possess a narrow therapeutic index—meaning the margin between a therapeutic, effective drug dose and a toxic dose is extremely small. TDM ensures that drug levels remain within the therapeutic window while avoiding severe toxicity.

Peak vs. Trough Concentrations

• Trough Level (Lowest Concentration): Represents the lowest concentration of the drug in the patient's bloodstream before the next dose.
  • Collection Timing: Drawn immediately before the next scheduled dose is administered (typically within a 15- to 30-minute window prior to infusion or oral dosing).
• Peak Level (Highest Concentration): Represents the maximum drug concentration achieved after administration.
  • Collection Timing: Depends strictly on the route of administration and drug pharmacokinetics:
    • Intravenous (IV) Infusion: Drawn 30 minutes after the IV infusion is completely finished.
    • Intramuscular (IM) Injection: Drawn 60 minutes after injection.
    • Oral Administration: Drawn 1 to 2 hours after oral ingestion.

Common TDM Medications

TDM is routinely performed for cardiac glycosides (Digoxin), aminoglycoside antibiotics (Gentamicin, Tobramycin), glycopeptide antibiotics (Vancomycin), anticonvulsants (Phenytoin, Valproic Acid), and bronchodilators (Theophylline).

---

Blood Bank Collections & Type and Crossmatch

Transfusion medicine draws represent the absolute highest risk area in phlebotomy practice. A single identification error in blood bank collection can lead to an ABO-incompatible blood transfusion, triggering acute intravascular hemolysis, anaphylaxis, renal failure, and death.

Strict Identification Protocols

• Zero-Tolerance Policy: There is zero tolerance for errors. Any discrepancy between the requisition, patient verbal statement, and patient ID band results in immediate rejection of the specimen.
• Dedicated Blood Tubes: Collected in a dedicated Pink Top EDTA tube (or Lavender EDTA depending on facility protocol) containing a special blood bank label.
• Secondary ID Wristband: In addition to the standard hospital ID wristband, the patient is fitted with a dedicated, tamper-evident Blood Bank ID Band featuring a unique alphanumeric identifier code.
• Verification Signatures: The phlebotomist must sign, date, record the time, and write their employee ID number on the tube label and blood bank form at the bedside, attesting that patient identity was physically verified.

---

Special Handling Requirements: Cold Agglutinins & Chain of Custody