Q: What is the most common cause of false-positive modified Allen test?
Answer: Over-extension of the wrist.
Modified Allen test is considered a requirement before inserting a radial arterial line but unfortunately, many factors may give false positive or false negative tests. Top three reasons for false-positive modified Allen test are an overextension of the wrist, skin tension over the ulnar artery, and an inexperienced operator.
Pulse oximetry and plethysmography are reasonable alternatives for more objective evaluation of palmar arch perfusion than modified Allen test.
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References:
1. Barbeau GR, Arsenault F, Dugas L, et al. Evaluation of the ulnopalmar arterial arches with pulse oximetry and plethysmography: comparison with the Allen's test in 1010 patients. Am Heart J 2004; 147:489.
2. Benit E, Vranckx P, Jaspers L, et al. Frequency of a positive modified Allen's test in 1,000 consecutive patients undergoing cardiac catheterization. Cathet Cardiovasc Diagn 1996; 38:352.
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