We read the letter by Fitzpatrick et al with interest.1 However, we note that their study employed 62 specimens from 19 patients; this implies that they utilised specimens that were not independent of each other. This is important as regression analysis and difference plots require specimens that are independent of each other. While this does not automatically imply that the findings in their letter are invalid, it is possible that the seven specimens with an overestimation of 36–40% were from either the same or a small number of patients and thus not be representative of the general population.
Footnotes
Competing interests: None declared.
References
- 1.Fitzpatrick F, McGaley T, Rajan L.et al Therapeutic drug monitoring of vancomycin in patients receiving haemodialysis: time for a change. J Clin Pathol 200659666–667. [PMC free article] [PubMed] [Google Scholar]