A patient undergoing treatment for epilepsy is admitted, whose blood sample for assessment of phenytoin levels is to be sent to laboratory. Which vacutainer should be used for collecting the blood sample?

Appeared in: RML Lucknow - 2021

Explanation

Phenytoin level is a therapeutic drug monitoring (TDM) test that requires a serum sample for analysis.
The Red cap vacutainer is a plain tube containing no anticoagulant, allowing blood to clot and produce serum.
This prevents interference from additives found in other tubes, ensuring an accurate measurement of the drug concentration.

Option A: The Blue cap vacutainer contains sodium citrate, an anticoagulant. It is used to collect plasma for coagulation studies like PT and aPTT, not for serum chemistry or TDM.
Option B: The Grey cap vacutainer contains potassium oxalate (an anticoagulant) and sodium fluoride (a glycolysis inhibitor). It is specifically designed for glucose and lactate testing.
Option D: The Yellow cap vacutainer can contain either SPS for blood cultures or a serum separator gel (SST). The gel in SST tubes can adsorb certain drugs, including phenytoin, leading to falsely low and inaccurate test results.

Clinical Relevance

Nursing practice connection: Use the key finding related to Phlebotomy tube selection for therapeutic drug monitoring (Phenytoin) to guide bedside assessment, documentation, and the next nursing action.
Correct tube selection is a critical nursing responsibility to ensure sample integrity and prevent diagnostic errors, repeat phlebotomy, and delays in treatment.
For phenytoin, which has a narrow therapeutic index (10-20 µg/mL), an accurate blood level is essential for dose adjustments to maintain efficacy and avoid toxicity.
Nurses must document the exact time of the blood draw and the time of the last dose to allow for proper interpretation of the results (e.g., trough vs. peak level).

How to Approach the Question

First, identify the core of the question: selecting the correct blood collection tube for a specific lab test (phenytoin level).
Recall that phenytoin is a medication, and measuring its level is a form of Therapeutic Drug Monitoring (TDM).
Remember that TDM and most chemistry tests require a serum sample, which is obtained from clotted blood.
Evaluate the options based on their additives and purpose: Blue (citrate for coagulation), Grey (fluoride/oxalate for glucose), Yellow (SPS for cultures or gel that can interfere), and Red (plain, for serum).
Conclude that the Red cap tube, which yields serum without interfering additives, is the correct choice for this test.

Concept Tested & Keywords

Concept Tested: Phlebotomy tube selection for therapeutic drug monitoring (Phenytoin)
Stem keywords: epilepsy, phenytoin levels, blood sample, vacutainer
Lead-in keywords: Which vacutainer
Negative lead-in flag: false

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E6 textbook of Applied Biochemistry and Nutrition & DieteticsHarbans Lal (pp 26-464 of 479) pp. 214-216, 213-215

E6 Pathology-Vandana Puri & Kavita Gaur Textbook of Pathology and Genetics p. 381-383

Practise the full RML Lucknow - 2021

Attempt every question from this paper in a timed mock, then review the full solution for each one.