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High Correlation Between Salivary Cortisol and Free Serum Cortisol Measurements Does Not Mean Good AgreementHigh Correlation Does Not Mean Good Agreement FREE TO VIEW

Jihad Mallat, MD
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From the Centre Hospitalier Dr Shaffner de Lens, Service de Reanimation polyvalente.

Correspondence to: Jihad Mallat, MD, Centre Hospitalier Dr Shaffner de Lens, Service de Reanimation polyvalente, Route de la Bassee, Lens 62300, France; e-mail: jihad_mallat@msn.com


Financial/nonfinancial disclosures: The author has reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (http://www.chestpubs.org/site/misc/reprints.xhtml).


© 2012 American College of Chest Physicians


Chest. 2012;141(1):273. doi:10.1378/chest.11-1959
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To the Editor:

I read with great interest the study by Estrada-Y-Martin and Orlander1 published recently in CHEST (November 2011). According to the study, salivary cortisol can replace free serum cortisol in patients with septic shock because of a good correlation between these variables. However, the correlation coefficient is of no practical use in the statistical analysis of comparison data.2 Indeed, the high correlation between salivary cortisol and free cortisol measurements does not mean that the two methods agree.3 The correlation coefficient in this case depends on both the variation between individuals and the variation within individuals (measurement error). Therefore, the correlation coefficient will partly depend on the choice of patients; if the variation between patients is high compared with the measurement error, the correlation will be high, whereas if the variation between patients is low, the correlation will be low no matter how good the agreement between the two methods.4 Consequently, when measurements are taken by two methods on a group of patients, the variation between patients will introduce a strong correlation between the two sets of measurements. This correlation is, of course, irrelevant for the method comparison. Thus, it is quite wrong to infer from high correlation that the salivary cortisol and free serum cortisol measurements may be used interchangeably.

A more appropriate approach would be to plot the difference between salivary cortisol and free serum cortisol measurements against their mean and to calculate the bias (the mean difference between the two measurements) and the 95% limits of agreement for the prediction interval for the differences as previously described by Bland and Altman.3 If this interval is sufficiently tight around 0 from a clinical point of view, salivary cortisol measurement can replace free serum cortisol measurement.

Inappropriate use of a regression analysis and correlation coefficient to the analysis of measurement method comparison data remains common in medicine, despite being well recognized. We should not forget that the correlation coefficient is a measure of association; it is not a measure of agreement.

Estrada-Y-Martin RM, Orlander PR. Salivary cortisol can replace free serum cortisol measurements in patients with septic shock. Chest. 2011;1405:1216-1222 [PubMed] [CrossRef]
 
Westgard JO, Hunt MR. Use and interpretation of common statistical tests in method-comparison studies. Clin Chem. 1973;191:49-57 [PubMed]
 
Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;18476:307-310 [PubMed]
 
Altman DG, Bland JM. Measurement in medicine: the analysis of method comparison studies. The Statistician. 1983;32:307-317
 

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References

Estrada-Y-Martin RM, Orlander PR. Salivary cortisol can replace free serum cortisol measurements in patients with septic shock. Chest. 2011;1405:1216-1222 [PubMed] [CrossRef]
 
Westgard JO, Hunt MR. Use and interpretation of common statistical tests in method-comparison studies. Clin Chem. 1973;191:49-57 [PubMed]
 
Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;18476:307-310 [PubMed]
 
Altman DG, Bland JM. Measurement in medicine: the analysis of method comparison studies. The Statistician. 1983;32:307-317
 
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