A patient undergoing treatment for epilepsy is advised to have a blood sample taken for assessment of phenytoin levels. Which vacutainer should be used in collecting the blood sample?

Appeared in: RML 2023

Explanation

Red cap vacutainers are used to collect serum for a wide range of laboratory tests, including serum biochemistry and serology.
Therapeutic Drug Monitoring (TDM), such as for phenytoin, is a type of biochemistry test that requires a serum sample.
It is critical to use a plain red-top tube that does not contain a serum separator gel.
Separator gels can absorb certain drugs, including phenytoin, which would lead to a falsely low reported drug level.
The therapeutic range for phenytoin is narrow (10-20 mcg/mL), making accurate measurement essential for patient safety.

Option A: The blue cap vacutainer contains sodium citrate, an anticoagulant. It is specifically used for coagulation studies like Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT), not for serum chemistry.
Option B: The grey cap vacutainer contains sodium fluoride, which inhibits glycolysis, and potassium oxalate, an anticoagulant. It is used exclusively for measuring glucose and lactate levels.
Option D: The yellow cap vacutainer contains Sodium Polyanethol Sulfonate (SPS), an anticoagulant that also inhibits complement activation. Its primary use is for collecting blood for microbiological cultures.

Clinical Relevance

Nursing practice connection: Use the key finding related to Phlebotomy: Correct vacutainer selection for therapeutic drug monitoring (Phenytoin) to guide bedside assessment, documentation, and the next nursing action.
Using the correct blood collection tube is a critical nursing responsibility that directly impacts patient safety and treatment accuracy.
An incorrect tube choice can lead to erroneous lab results. For a drug like phenytoin, a falsely low level (from using a gel tube) might prompt a dose increase, risking toxicity (symptoms: nystagmus, ataxia, confusion).
Conversely, a falsely high level could lead to a dose reduction, risking sub-therapeutic levels and breakthrough seizures.

How to Approach the Question

First, identify the specific laboratory test being ordered: 'assessment of phenytoin levels'.
Recognize this test as a form of Therapeutic Drug Monitoring (TDM), which is a sub-category of serum biochemistry.
Recall the standard vacutainer tube colors and their primary functions from your nursing knowledge.
Match the test requirement (serum for biochemistry) with the correct tube. The red-top tube is the standard choice for obtaining serum.
Consider any special precautions. For certain drugs like phenytoin, it's important to avoid serum separator gels, which reinforces the choice of a plain red-top tube.
Systematically eliminate the other options by recalling their specific uses: blue for coagulation, grey for glucose, and yellow for blood cultures.

Concept Tested & Keywords

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

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

E6 Pathology-Vandana Puri & Kavita Gaur Textbook of Pathology and Genetics p. 380-382

E6 Pharmacology Nursing Lilley 11e Part 1 p. 241-243

Practise the full RML 2023

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