Precision in Practice: A Guide to Laboratory Specimen Collection

In clinical diagnostics, the journey from patient to result begins with the specimen. A laboratory result is only as good as the sample provided; improper collection, handling, or transport can lead to hemolysis, contamination, or degradation, ultimately affecting patient care. This guide outlines the gold standards for collecting blood, urine, stool, and swab specimens, along with the crucial logistics of transport and chain of custody.

1. Blood Collection: The Art of Phlebotomy

Phlebotomy is the most common invasive procedure in healthcare. Success depends on technique, patient preparation, and adherence to safety protocols.

Best Phlebotomy Techniques

• Patient Identification: Always use two identifiers (e.g., full name and date of birth) before beginning.
• Site Selection: The median cubital vein is the preferred site. Avoid sites with hematomas, edema, or IV lines (if unavoidable, draw distal to the IV line after turning it off for at least 2 minutes).
• Tourniquet Application: Apply 3–4 inches above the site. Do not leave it on for more than 1 minute to prevent hemoconcentration, which can skew potassium and protein levels.
• The Draw: Anchor the vein firmly with your thumb below the site to prevent rolling. Insert the needle at a 15–30 degree angle.
• Mixing: Gently invert tubes 5–10 times immediately after collection. Never shake the tubes, as this causes hemolysis (rupture of red blood cells).

Order of Draw (Vacutainers)

Following the correct order of draw is vital to prevent cross-contamination of additives between tubes.

1. Plain: (Red top)
2. Blood Cultures: (Yellow top or bottles) - Drawn first to minimize bacterial contamination.
   
3. Sodium Citrate: (Light Blue) - For coagulation studies (PT/INR). Must be filled to the line.
4. Serum Tubes: (Red, Gold/SST, Tiger Top) - For chemistry panels, serology, and immunology.
5. Heparin: (Green) - For plasma chemistry determinations.
6. EDTA: (Lavender/Purple) - For Hematology (CBC) and Blood Bank.
7. Oxalate/Fluoride: (Gray) - For glucose and lactate testing.

2. Urine Collection

Urine specimens are sensitive to bacterial overgrowth if not handled correctly.

• Random Specimen: Used for routine screening. No specific preparation is required, though the "midstream" technique is preferred to reduce contaminants.
• Clean-Catch Midstream: Essential for cultures. The patient must clean the periurethral area with antiseptic wipes. They should void a small amount into the toilet, stop, and then collect the sample in the sterile cup without touching the inside of the container.
• 24-Hour Collection: Used for quantitative analysis (e.g., protein, creatinine clearance). The first morning void is discarded, and all subsequent urine is collected for the next 24 hours, including the next morning's first void. These jugs often require preservatives or refrigeration.

3. Stool Collection

Stool samples are used to detect parasites, occult blood, and enteric pathogens.

• Technique: Patients should avoid contaminating the stool with urine or toilet water. A "hat" collection device placed under the toilet seat is often used.
• O&P (Ova and Parasites): Requires immediate transfer into vials containing preservatives (like formalin or PVA) to maintain parasite structure.
• C. difficile/Culture: Fresh stool is required. If testing for C. diff toxin, the sample must be loose or liquid; formed stool is generally rejected.

4. Swabs: Virology vs. Bacteriology

Not all swabs are created equal. Using the wrong shaft material or transport medium can kill the organism you are trying to detect.

• Bacteriology (Aerobic/Anaerobic):
  • Amies or Stuart Transport Media: Gel or liquid media that keeps bacteria alive but prevents overgrowth.
  • Wound Swabs: Clean the wound surface with saline first to remove debris, then swab deep into the wound bed, avoiding the skin edges.
• Virology (Viral Transport Media - VTM):
  • Dacron or Rayon tips: Cotton and calcium alginate swabs can inactivate some viruses and inhibit PCR reactions.
  • Universal Transport Media (UTM): Used for viral culture, rapid antigen tests, and PCR (e.g., Influenza, COVID-19).
• Nasopharyngeal (NP) Swabs: The gold standard for respiratory viruses. The swab must be inserted parallel to the palate (not upwards) until resistance is met, rotated, and held for several seconds to absorb secretions.

5. Specimen Transportation and Conservation

Once collected, the clock starts ticking.

• Temperature Control:
  • Room Temperature (15-25°C): Most blood cultures and anaerobic bacteria swabs.
  • Refrigerated (2-8°C): Urine (if not tested within 2 hours), blood tubes for certain chemistries (like ammonia, which also requires ice slurry), and most viral transport media.
  • Frozen (-20°C or lower): Serum or plasma for special chemistry or molecular testing if there is a delay in transport.
• Protection from Extremes:
  • Heat: High temperatures can denature proteins and kill fastidious bacteria. Courier cars must have climate control or insulated cooler boxes.
  • Cold: Freezing whole blood causes massive hemolysis, rendering the sample useless for most tests. Insulated containers are vital during winter transport.

6. Chain of Custody

For legal or forensic testing (e.g., drug screening, paternity testing, blood alcohol for legal proceedings), a strict Chain of Custody (CoC) must be maintained.

• Documentation: Every individual who handles the specimen must sign and date the CoC form. There can be no gaps in the timeline.
• Security: The specimen must be sealed with a tamper-evident tape in the presence of the donor.
• Storage: If the specimen cannot be tested immediately, it must be locked in a secure refrigerator or storage area accessible only to authorized personnel.

Disclaimer: This guide is for educational purposes. Always refer to your specific laboratory’s standard operating procedures (SOPs) and manufacturer instructions for collection devices.