John C.Dalrymple-Alford, Associate Professor, Van der Veer Institute for Parkinson's and Brain Researchjohn.dalrymple-alford@canterbury.ac.nz
C.T. Nakas, PhD, M.R. MacAskill, PhD, L. Livingston, BA, and T.J. Anderson, MD
Submitted February 02, 2011
On reading our study of screening instruments for cognitive impairment in PD, [1] Marinus et al. [2] suggested that we made an inappropriate comparison between the validity of the MoCA and their instrument, the SCOPA-COG. They correctly noted that these instruments were not administered to all patients.
The critical issue is their relative value in discriminating PD patients with relatively normal cognition (PD-N) from those meeting our criteria for Mild Cognitive Impairment (PD-MCI). Although not explicitly stated in our article, comparisons between the area under the curve (AUC) of any two instruments requires restriction to those individuals tested on both measures and this comparison was reported. The two instruments were equivalent when assessing dementia (PD-D) relative to no dementia, but a difference emerged in favor of the MoCA when examining the separation between PD-N and PD-MCI.
As we stated, the AUC difference in favor of the MoCA was 12% (95% confidence interval [CI], 0.3-24.0%, p=0.045, p.1720). [1] The AUC for the MoCA itself for those PD-MCI and PD-N patients tested on both MoCA and SCOPA-COG was 93% (CI, 83-98%). Therefore, the AUC was at least marginally better than when examining all patients tested on the MoCA (90%; CI, 82-95%). It is unlikely that the specificity of the SCOPA-COG for the PD-N versus PD-MCI comparison would improve with a larger sample for a screening cut-off because this instrument is sensitive to minor impairments evident in the PD-N group (Table 2). [1]
Relative to the values given for the whole sample in Table 3, [1] no diagnostic performance values worsened and some improved when the MoCA analysis was restricted to those PD-N and PD-MCI patients who were tested on both the MoCA and the SCOPA-COG. The cut-offs suggested for optimal screen, diagnostics, and maximum accuracy remained the same. However, specificity for the optimal screen cut-off increased from 75% to 86% while positive predictive value (PPV) increased from 61% to 73%; the PPV for the optimal diagnostic cut-off increased from 79% to 90%; and both specificity (75% to 86%) and PPV (61% to 73%) increased for the maximum accuracy cut- off. The base-rates used to estimate PPV and NPV were population base- rates, not sample base-rates, which will vary across different criteria especially in PD-MCI. [3]
This evidence suggests that our original conclusion of the value of MoCA regarding cognitive screening for PD is sound.
Reference
3. Dalrymple-Alford JC, Livingston L, MacAskill MR, Graham C et al. Characterizing mild cognitive impairment in Parkinson's disease. Movement Disorders. doi: 10.1002/mds.23592. published online: 1 FEB 2011.
Disclosures: See original article for full disclosure list.
On reading our study of screening instruments for cognitive impairment in PD, [1] Marinus et al. [2] suggested that we made an inappropriate comparison between the validity of the MoCA and their instrument, the SCOPA-COG. They correctly noted that these instruments were not administered to all patients.
The critical issue is their relative value in discriminating PD patients with relatively normal cognition (PD-N) from those meeting our criteria for Mild Cognitive Impairment (PD-MCI). Although not explicitly stated in our article, comparisons between the area under the curve (AUC) of any two instruments requires restriction to those individuals tested on both measures and this comparison was reported. The two instruments were equivalent when assessing dementia (PD-D) relative to no dementia, but a difference emerged in favor of the MoCA when examining the separation between PD-N and PD-MCI.
As we stated, the AUC difference in favor of the MoCA was 12% (95% confidence interval [CI], 0.3-24.0%, p=0.045, p.1720). [1] The AUC for the MoCA itself for those PD-MCI and PD-N patients tested on both MoCA and SCOPA-COG was 93% (CI, 83-98%). Therefore, the AUC was at least marginally better than when examining all patients tested on the MoCA (90%; CI, 82-95%). It is unlikely that the specificity of the SCOPA-COG for the PD-N versus PD-MCI comparison would improve with a larger sample for a screening cut-off because this instrument is sensitive to minor impairments evident in the PD-N group (Table 2). [1]
Relative to the values given for the whole sample in Table 3, [1] no diagnostic performance values worsened and some improved when the MoCA analysis was restricted to those PD-N and PD-MCI patients who were tested on both the MoCA and the SCOPA-COG. The cut-offs suggested for optimal screen, diagnostics, and maximum accuracy remained the same. However, specificity for the optimal screen cut-off increased from 75% to 86% while positive predictive value (PPV) increased from 61% to 73%; the PPV for the optimal diagnostic cut-off increased from 79% to 90%; and both specificity (75% to 86%) and PPV (61% to 73%) increased for the maximum accuracy cut- off. The base-rates used to estimate PPV and NPV were population base- rates, not sample base-rates, which will vary across different criteria especially in PD-MCI. [3]
This evidence suggests that our original conclusion of the value of MoCA regarding cognitive screening for PD is sound.
Reference
3. Dalrymple-Alford JC, Livingston L, MacAskill MR, Graham C et al. Characterizing mild cognitive impairment in Parkinson's disease. Movement Disorders. doi: 10.1002/mds.23592. published online: 1 FEB 2011.
Disclosures: See original article for full disclosure list.