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To the Editor.-Reply

Mitchell L. Rhodes, MD
Arch Intern Med. 1980;140(3):440-441. doi:10.1001/archinte.1980.00330150154042.
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I appreciate the opportunity to clarify some misunderstandings of interpretation of our article "Relationship Between Arterial and Venous Bicarbonate Values," as expressed in the letters of Drs Riff, Westhoven, and Marshall. Drs Westhoven and Riff point out the linear correlation found between arterial bicarbonate, calculated from the Henderson-Hasselbalch equation, and measured venous CO,. Dr Riff asked why one cannot substitute the venous value for the arterial value in the equation since it is "virtually identical." We never questioned the validity of the equation as Dr Riff implies. Examination of the shape of the 95% confidence interval on page 441 of our article demonstrates that in the 336 patients with altered arterial bicarbonate values, either above or below normal, the figure progressively widens, indicating greater scatter (nonlinearity) in the relationship to venous CO2. Our points were that one should not try to perform quality control checks on the blood gas


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