Neurology -- Correspondence for Suchowersky et al., 66 (7) 968-975

Correspondence published:

Practice Parameter: Diagnosis and prognosis of new onset Parkinson disease (an evidence-based review: Erwin B. Montgomery Jr. (21 June 2006)

Reply from the Authors: O. Suchowersky, G. Gronseth, MD, University of Kansas, W. Weiner, MD, University of Maryland (21 June 2006)

Practice Parameter: Diagnosis and prognosis of new onset Parkinson disease (an evidence-based review 21 June 2006

Erwin B. Montgomery Jr.,
University of Wisconsin - Madison
H6/538 CSC, 600 Highland Ave., Madison WI 53792
Send Correspondence to journal:
Re: Practice Parameter: Diagnosis and prognosis of new onset Parkinson disease (an evidence-based review

montgomery{at}neurology.wisc.edu Erwin B. Montgomery Jr.

While an excellent review, the conclusions of the guidelines by Suchowersky et al [1] are flawed because specificities and sensitivities were considered and not positive and negative predictive values. The latter incorporate the prevalence of the conditions being diagnosed and the former do not. The implications are significant.
Consider a sample of 100 patients with Parkinsonism. Assuming 20% prevalence of subjects have other than idiopathic Parkinson’s disease (iPD) as suggested in the guidelines, a test with 80% specificity and 80% sensitivity would correctly identify 64 of the 80 subjects with iPD and falsely identify 16 as having something other than iPD (false negatives for a diagnosis of iPD). Of the 20 subjects with other than iPD, the test would correctly 16 of the 20 has having other than iPD and would incorrectly identify 4 of the 20 as having iPD (false positives).
Using this test to treat only persons with iPD based on the diagnostic test would incorrectly withhold treatment from four times as many patients with iPD than would incorrectly treat those with other than iPD. The situation would be worse if the prevalence of other than iPD was approximately 8% as suggested by the retrospective analysis of the DATATOP study. [2] Yet the levodopa challenge test was reported to have a sensitivity of 70.9% and a specificity of 81.4% and was described in the guidelines as “probably useful in distinguishing PD from other parkinsonian syndromes.” The ratio of false positives to false negatives based on 80% specificity and sensitivity would not necessarily be bad depending on the consequences.
If the treatment had low risk and cost, then the social, moral, ethical, medical and economic consequences of not treating the false negatives might be worse than treating the false positives. If the treatment had high risk and high cost, then the social, moral, ethical, medical and economic consequences would be very different.
The value of any diagnostic test cannot be truly evaluated independent of the social, moral, ethical, medical and economic consequences of the decisions that follow from the application of the test. [3] Unfortunately, these consequences or even the importance of considering such consequences were not addressed in the guidelines and demonstrate the limitations of recommendations based exclusively on current uses of evidence based medicine, which are quite different from what was originally intended. [3]
References
1. Suchowersky O, Reich S, Perlmutter J, Zesiewicz T, Gronseth G, Weiner WJ. Practice Parameter: Diagnosis and prognosis of new onset Parkinson disease (an evidence-based review): Report of the Quality Standards Subcommittee of the American Academy of Neurology. Neurology 2006;66:968-975.
2. Jankovic J, Rajput AH, McDermott MP, Perl DP. The evolution of diagnosis in early Parkinson disease. Parkinson Study Group. Arch Neurol 2000;57:369-372.
3. Montgomery Jr. EB, Turkstra LS. Evidenced based medicine: let’s be reasonable. J Med Speech Lang Path 2003;11:ix-xii.
Disclosure: The author reports no conflicts of interest.

Reply from the Authors 21 June 2006

O. Suchowersky,
University of Calgary ,
G. Gronseth, MD, University of Kansas, W. Weiner, MD, University of Maryland
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Re: Reply from the Authors

osuchowe{at}ucalgary.ca O. Suchowersky, et al.

We thank Dr. Montgomery for his thoughtful comments. Dr. Montgomery suggests that we should have used positive and negative predictive values rather than specificities and sensitivities in assessing diagnostic accuracy. However, positive and negative predictive values vary considerably based on the prevalence of the disease in a specific study’s population. Thus, they may be misleading when applied to a clinical situation. In these circumstances, as nicely illustrated by Dr. Montgomery, the sensitivity and specificity may be used to calculate the positive and negative predictive values of a test.
Secondly, our paper [1] does not make any recommendations or even suggest that treatment with dopaminergic medications should be withheld from individuals with Parkinsonism that do not fit the diagnostic criteria. Appropriate treatment is best decided by the treating physician.
Lastly, Dr. Montgomery states that ethical, moral and social factors are important in diagnostic testing and treatment. We agree with his comments; these are important factors that should be left to the judgment of the physician. The practice parameter is a useful adjunct in this decision-making process.
Disclosure: The authors report no conflicts of interest.