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Allen's Test Ulnar Collateral — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementAllen's Test Ulnar CollateralFRCA Primary

Before radial cannulation, release of ulnar compression during a plethysmographic modified Allen test fails to restore the waveform. What is the most defensible interpretation?

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Correct answer: B — Collateral perfusion appears inadequate; consider another site

The best answer is “Collateral perfusion appears inadequate; consider another site”. Failure of plethysmographic recovery suggests inadequate collateral flow and supports choosing another site or further vascular assessment. The test does not prove future ischaemia and does not localise an occlusion with certainty. A warm hand at baseline cannot substitute for demonstrating collateral perfusion when the radial supply is compressed.

Reference: Saugel et al., How to measure blood pressure using an arterial catheter: a systematic 5-step approach: https://pmc.ncbi.nlm.nih.gov/articles/PMC7183114/