Unit 4: Special Collection Procedures

• Topic 01

  Capillary Collection — Fingerstick Technique, Lancet Selection, and Heelstick

  15 questions
• Topic 02

  Paediatric Venepuncture — Scalp Veins and Small Vein Techniques

  15 questions
• Topic 03

  Blood Cultures — Aerobic/Anaerobic Bottles, Aseptic Technique, and Volume

  16 questions
• Topic 04

  Arterial Blood Gas Collection — Radial Puncture and Allen Test

  39 questions
• Topic 05

  Glucose Tolerance Test (GTT) — Timing and Patient Preparation

  15 questions
• Topic 06

  Therapeutic Drug Monitoring — Peak and Trough Specimen Timing

  15 questions
• Topic 07

  Newborn Screening (PKU/Heel Stick Card) Technique

  15 questions

A few questions from this unit, with the answer and a full explanation. The complete bank is available when you start practising.

1. Why must a trough specimen be drawn immediately before the next scheduled dose?

   • A
     The timing only matters for drugs with half-lives longer than 12 hours; for shorter half-life drugs any pre-dose time is acceptable
   • B
     The trough represents the lowest drug concentration just before the next dose; drawing it too early produces a falsely elevated result that may lead to an unnecessary dose reduction
     Correct answer
   • C
     Trough specimens drawn earlier in the interval are more accurate because the drug has fully distributed into tissues
   • D
     The trough must be drawn at the dose time so that the nurse and phlebotomist can coordinate the collection and administration simultaneously
   Explanation

   Trough levels are meaningful only if they reflect the true nadir (lowest point) in the dosing cycle. If drawn 1–2 hours before the next dose, the drug has not yet fallen to its minimum — the result is falsely elevated. A pharmacist using this result may reduce the dose unnecessarily, leading to subtherapeutic levels and treatment failure. ISMP classifies trough timing errors as a high-alert medication safety concern. Key takeaway: Trough drawn early gives a falsely elevated result that may cause the dose to be reduced unnecessarily.

2. How does residual air in an ABG syringe affect PO2 and PCO2 values?

   • A
     Residual air lowers PO2 and raises PCO2 because blood absorbs oxygen from air in the syringe
   • B
     Residual air only affects pH, not PO2 or PCO2 values
   • C
     Residual air lowers both PO2 and PCO2 by diluting the blood sample with atmospheric gas
   • D
     Residual air raises PO2 and lowers PCO2 because room air has higher oxygen and lower carbon dioxide concentrations than blood
     Correct answer
   Explanation

   Room air contains approximately 21% oxygen (PO2 ~150 mmHg) and 0.04% CO2 (PCO2 ~0.3 mmHg). Blood PO2 is typically 80–100 mmHg and PCO2 is 35–45 mmHg. When air equilibrates with the specimen, it pulls PO2 up and PaCO2 down. These changes can lead to incorrect clinical decisions, particularly in hypoxic patients. Key takeaway: Air in the syringe falsely elevates PO2 and falsely decreases PCO2.

3. A phlebotomist is collecting capillary blood from an adult patient whose middle and ring fingers are swollen from an IV infiltration. The patient has calluses on both thumbs from manual labor. Which of the following site selections is most appropriate?

   • A
     Use the thumb of the dominant hand because its larger size compensates for the callus and arterial concerns
   • B
     Proceed with the swollen finger because edema increases capillary blood pressure and improves blood flow
   • C
     Use the non-dominant hand's ring finger or middle finger if one is less swollen, or assess the 5th finger of the non-affected hand if tissue depth is adequate, avoiding the thumb due to pulsation risk
     Correct answer
   • D
     Switch to venipuncture without further assessment, as no capillary site is usable in this scenario
   Explanation

   Swollen or edematous fingers must be avoided because excess interstitial fluid dilutes the capillary specimen (hemodilution). Callused thumbs should be avoided due to pulsation risk (arterial puncture) and thick skin impairing flow. The phlebotomist should find the least-compromised digit on the less-affected hand, prioritizing the 3rd or 4th finger. Key takeaway: Avoid swollen (hemodilution risk) and thumb (arterial/callus risk) sites — find the best available alternative digit.