A Bland-Altman plot is constructed to compare a new portable spirometer with the gold-standard lab spirometer for FEV1 measurements. The mean difference (bias) is +0.12 L and the standard deviation of differences is 0.15 L. What are the 95% limits of agreement?
- A −0.03 L to +0.27 L
- B −0.12 L to +0.36 L
- C −0.18 L to +0.42 L ✓
- D 0.00 L to +0.24 L
Correct answer: C. −0.18 L to +0.42 L
Explanation
The 95% limits of agreement in a Bland-Altman analysis are calculated as mean difference ± 1.96 × SD = 0.12 ± 1.96 × 0.15 = 0.12 ± 0.294, giving −0.174 ≈ −0.18 to +0.414 ≈ +0.42. The Bland-Altman method assesses method agreement, not correlation; a wide spread indicates the new device is not interchangeable with the gold standard.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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