Neurourology and Urodynamics 26:454 (2007) LETTER TO THE EDITOR Re: Griffiths C, Drinnan M, Harding C, et al. 2005. Comparison of Invasive and Non-Invasive Bladder Pressure Measurements by Calculation of the Bladder Outlet Obstruction Index. Neurourol Urodynam 24:529–531 To the Editor: For many years, urologists have been searching for a non- invasive urodynamic technique. Recently, Griffiths et al. 1,2 made more effort on penile cuff pressure, which was developed a decade ago. 3 Griffiths et al., plotted and analyzed the data by using the technique of Bland and Altman 4 and concluded that ‘‘bladder outlet obstruction index derived from non-invasive data is in moderate agreement with invasive measurement.’’ We could not agree more that the standard deviation of the differences between measurements made by two methods provides a good index of the comparability of the methods. If the mean and standard deviation are reliable, with small standard errors, then the difference between the methods will be at most two standard deviations on either side of the mean for 95% of observation, called the 95% limits of agreement. In Griffiths’s article, 2 the mean value and the standard deviation of the difference between invasive and non-invasive methods are 14.0 and 36.1. To us the variation of the difference between invasive and noninvasive methods seems sufficiently large. There- fore, the paired t-test should be further investigated before making any conclusion. Furthermore, it might be not a bad idea to investigate the correlation between the pressures measured by the invasive and noninvasive methods. The correlation 5 can tell us how close the relationship between the two methods is. In other words, without further statistical analysis, it seems difficult to conclude that ‘‘the bladder outlet obstruction index derived from non- invasive data is in moderate agreement with invasive measurement.’’ REFERENCES 1. Griffiths C, Blake C, Harding C, et al. 2003. Non-invasive bladder pressure: The case for using a modified ICS nomgram. Neurourol Urodynam 22:367–8. 2. Griffiths C, Drinnan M, Harding C, et al. 2005. Comparison of invasive and non-invasive bladder pressure measurements by calculation of the bladder outlet obstruction index. Neurourol Urodynam 24:529–531. 3. Schafer W, Kirschner-hermans R, Jakes G. 1994. Non-invasive pressure/flow measurement for precise grading of bladder outflow obstruction. J Urol 151:323A. 4. Bland JM, Altman DG. 1986. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet I:307–10. 5. Bland M. 2000. An introduction to medical statistics, 3rd edn. New York: Oxford University Press. Shyh-Jen Wang* Division of Experimental Surgery Department of Surgery Veterans General Hospital-Taipei Institute of Bio-Engineering National Yang-Ming University Taipei, Taiwan, Republic of China Alex Tong-Long Lin Division of Urology Surgery Department of Surgery Veterans General Hospital-Taipei Department of Urology National Yang-Ming University Taipei, Taiwan, Republic of China Chao-An Lin Cheng-Hsiu Yang Department of Power Mechanical Engineering National Tsing-Hau University Taipei, Taiwan, Republic of China No conflict of interest reported by the author(s). *Correspondence to: Shyh-Jen Wang, PhD, PE, Division of Experimental Surgery, Department of Surgery, Veterans General Hospital-Taipei, Institute of Bio- Engineering, National Yang-Ming University, Taipei, Taiwan, Republic of China. E-mail: [email protected] Published online 29 January 2007 in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/nau.20237 ß 2007 Wiley-Liss, Inc.

Re: Griffiths C, Drinnan M, Harding C, et al. 2005. Comparison of invasive and non-invasive bladder pressure measurements by calculation of the bladder outlet obstruction index. Neurourol

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Page 1: Re: Griffiths C, Drinnan M, Harding C, et al. 2005. Comparison of invasive and non-invasive bladder pressure measurements by calculation of the bladder outlet obstruction index. Neurourol

Neurourology and Urodynamics 26:454 (2007)

LETTER TO THE EDITOR

Re: Griffiths C, Drinnan M, Harding C, et al. 2005. Comparisonof Invasive and Non-Invasive Bladder Pressure Measurements by

Calculation of the Bladder Outlet Obstruction Index.Neurourol Urodynam 24:529–531

To the Editor:

For many years, urologists have been searching for a non-invasive urodynamic technique. Recently, Griffiths et al.1,2

made more effort on penile cuff pressure, which wasdeveloped a decade ago.3 Griffiths et al., plotted and analyzedthe data by using the technique of Bland and Altman4 andconcluded that ‘‘bladder outlet obstruction index derived fromnon-invasive data is in moderate agreement with invasivemeasurement.’’

We could not agree more that the standard deviation of thedifferences between measurements made by two methodsprovides a good index of the comparability of the methods. Ifthe mean and standard deviation are reliable, with smallstandard errors, then the difference between the methods willbe at most two standard deviations on either side of the meanfor 95% of observation, called the 95% limits of agreement. InGriffiths’s article,2 the mean value and the standard deviationof the difference between invasive and non-invasive methodsare �14.0 and 36.1.

To us the variation of the difference between invasiveand noninvasive methods seems sufficiently large. There-fore, the paired t-test should be further investigatedbefore making any conclusion. Furthermore, it might be nota bad idea to investigate the correlation between thepressures measured by the invasive and noninvasive meth-ods. The correlation5 can tell us how close the relationshipbetween the two methods is. In other words, withoutfurther statistical analysis, it seems difficult to conclude that‘‘the bladder outlet obstruction index derived from non-invasive data is in moderate agreement with invasivemeasurement.’’

REFERENCES

1. Griffiths C, Blake C, Harding C, et al. 2003. Non-invasive bladder pressure: Thecase for using a modified ICS nomgram. Neurourol Urodynam 22:367–8.

2. Griffiths C, Drinnan M, Harding C, et al. 2005. Comparison of invasive andnon-invasive bladder pressure measurements by calculation of the bladderoutlet obstruction index. Neurourol Urodynam 24:529–531.

3. Schafer W, Kirschner-hermans R, Jakes G. 1994. Non-invasive pressure/flowmeasurement for precise grading of bladder outflow obstruction. J Urol151:323A.

4. Bland JM, Altman DG. 1986. Statistical methods for assessing agreementbetween two methods of clinical measurement. Lancet I:307–10.

5. Bland M. 2000. An introduction to medical statistics, 3rd edn. New York:Oxford University Press.

Shyh-Jen Wang*Division of Experimental Surgery

Department of SurgeryVeterans General Hospital-Taipei

Institute of Bio-EngineeringNational Yang-Ming University

Taipei, Taiwan, Republic of China

Alex Tong-Long LinDivision of Urology Surgery

Department of SurgeryVeterans General Hospital-Taipei

Department of UrologyNational Yang-Ming University

Taipei, Taiwan, Republic of China

Chao-An LinCheng-Hsiu Yang

Department of Power Mechanical EngineeringNational Tsing-Hau University

Taipei, Taiwan, Republic of China

No conflict of interest reported by the author(s).*Correspondence to: Shyh-Jen Wang, PhD, PE, Division of Experimental Surgery,Department of Surgery, Veterans General Hospital-Taipei, Institute of Bio-Engineering, National Yang-Ming University, Taipei, Taiwan, Republic of China.E-mail: [email protected] online 29 January 2007 in Wiley InterScience(www.interscience.wiley.com)DOI 10.1002/nau.20237

� 2007 Wiley-Liss, Inc.