However, an additional confounding variable must be considered as it has the potential to introduce further significant error. PBW equations all use patient height, and it is our experience and that of others that accurate measurement of this parameter in critically ill patients often can be extremely difficult. Current practice is frequently to use head-to-heel length, measured while the patient is supine in the bed; however, this method has been shown to overestimate height significantly. Significant variation also exists among health-care professionals performing measurements. Numerous alternative methods of deriving height have been described and validated, particularly in the elderly population. Researchers use formulas that require measuring arm span, arm demi-span, knee height, or ulnar length,, which may be easier to perform. Not all of these methods have been validated in patients who are receiving critical care, and the optimal method of height measurement in this patient group remains to be determined.