Which of the following additive used in a blood sample vial for Prothrombin time?

NORCET 5 Prelims - Sept 2023 (Shift-2)

Biochemistry and Nutrition

Easy

Appeared in: NORCET 5 Prelims - Sept 2023 (Shift-2)

Explanation

Sodium citrate is the standard anticoagulant for coagulation studies, including Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT).
It functions by reversibly binding (chelating) calcium ions in the blood, which are essential for the clotting cascade to proceed.
This action preserves the coagulation factors in the sample, allowing for an accurate measurement of clotting time once the sample reaches the laboratory.
Blood for PT is collected in a light blue-top tube, which contains a specific concentration of sodium citrate, requiring a 9:1 ratio of blood to anticoagulant for accuracy.

Option B: Oxalate (usually potassium oxalate) is an anticoagulant, but it is primarily used in gray-top tubes for glucose and lactate testing because it is combined with sodium fluoride, which inhibits glycolysis.
Option C: Heparin is a potent anticoagulant that works by inhibiting thrombin. Its direct interference with the clotting cascade makes it unsuitable for coagulation tests like PT, as it would falsely prolong the results.
Option D: EDTA (Ethylenediaminetetraacetic acid) is a strong anticoagulant that irreversibly binds calcium. It is the standard for hematology (in lavender-top tubes) because it preserves the morphology of blood cells for a Complete Blood Count (CBC), but it is not used for coagulation studies.

Visual 1: Diagram - An infographic showing the different types of blood collection vacutainer tubes with their corresponding cap colors, additives, and primary laboratory uses. This helps in visually associating the light-blue top tube with Sodium Citrate and coagulation tests.

Clinical Relevance

Nursing practice connection: Knowing Anticoagulants for Laboratory Blood Tests helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Using the correct blood collection tube is a critical patient safety measure. An incorrect PT result due to the wrong additive can lead to improper dosing of anticoagulant medications like warfarin, increasing the risk of either life-threatening bleeding or thrombosis.
Nurses are responsible for ensuring the tube is filled to the correct volume. The 9:1 blood-to-additive ratio in citrate tubes is crucial; under-filling or over-filling the tube will alter this ratio and lead to erroneous PT/INR results.
What if? - If a lavender-top (EDTA) tube was accidentally sent for a PT test, the lab would likely reject the sample. If a result were obtained, it would be falsely prolonged because EDTA binds calcium much more strongly than citrate, preventing clotting even after the lab adds its reagents.

How to Approach the Question

First, identify the key terms in the question: 'additive' and 'Prothrombin time (PT)'.
Recognize that Prothrombin Time is a fundamental test of the blood's clotting ability, i.e., a coagulation study.
Recall the standard color-coded system for blood collection tubes and the specific anticoagulant in each.
Mentally link the light blue-top tube to coagulation studies.
Remember that the additive in the light blue-top tube is Sodium Citrate.
Evaluate the other options: EDTA (lavender top) is for hematology/CBC, Heparin (green top) is for chemistry, and Oxalate (gray top) is for glucose. This confirms they are incorrect for a PT test.

Concept Tested & Keywords

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