The only published data on indices that I am aware of, is in the Annals publication ("Anti–JC Virus Antibody Levels in Serum or Plasma Further Define Risk of Natalizumab-Associated Progressive Multifocal Leukoencephalopathy", ANN NEUROL 2014;76:802–812). A couple of quotes that may be relevant to this discussion are pasted below. If anyone has information on PML in 'low titer positives' (index <0.9), I would appreciate receiving it. The official Biogen website, that gives updates on PML cases (https://medinfo.biogenidec.com/, accessed today) has this to say: "As of Mar 3, 2015, there are 278 natalizumab-treated MS PML patients with pre-PML samples collected at least 6 months prior to PML diagnosis. Of these 278 patients, 275 (99%) tested anti-JCV antibody positive prior to diagnosis and 3 (1%) tested anti-JCV antibody negative." However, there is no information on PML in low-positives.
Quotes from Annals article:
"Using the predicted probabilities from the combined data sets, PML risk estimates were generated for anti-JCV antibody index thresholds of 0.9 to 1.5 (Table 2). For anti-JCV antibody–positive patients with no prior immunosuppressant use and an anti-JCV antibody index at or below thresholds of 0.9 to 1.5, the risk of PML was approximately 0.1 per 1,000 patients during the first 2 years of natalizumab treatment, and it ranged from 0.3 to 1.3 per 1,000 patients from month 25 to 48 and from month 49 to 72. For patients with no prior immunosuppressant use and an anti-JCV antibody index > 1.5, the risk of PML was approximately 1 per 1,000 patients during the first 2 years of natalizumab treatment, and ranged from 8.1 to 8.5 per 1,000 patients from month 25 to 48 and from month 49 to 72.
...
Twenty-five natalizumab-treated MS patients with no prior immunosuppressant use who developed PML had at least 2 pre-PML samples. For 24 of these patients (96%), all samples had an anti-JCV antibody index- > 0.9, and for 21 of 25 patients (84%), all samples had an anti-JCV antibody index > 1.5 (Fig 5). In 1 patient, 3 of 4 available samples had an anti-JCV antibody index- 0.9, 2 of which were collected within 12 months of PML diagnosis."
Quotes from Annals article:
"Using the predicted probabilities from the combined data sets, PML risk estimates were generated for anti-JCV antibody index thresholds of 0.9 to 1.5 (Table 2). For anti-JCV antibody–positive patients with no prior immunosuppressant use and an anti-JCV antibody index at or below thresholds of 0.9 to 1.5, the risk of PML was approximately 0.1 per 1,000 patients during the first 2 years of natalizumab treatment, and it ranged from 0.3 to 1.3 per 1,000 patients from month 25 to 48 and from month 49 to 72. For patients with no prior immunosuppressant use and an anti-JCV antibody index > 1.5, the risk of PML was approximately 1 per 1,000 patients during the first 2 years of natalizumab treatment, and ranged from 8.1 to 8.5 per 1,000 patients from month 25 to 48 and from month 49 to 72.
...
Twenty-five natalizumab-treated MS patients with no prior immunosuppressant use who developed PML had at least 2 pre-PML samples. For 24 of these patients (96%), all samples had an anti-JCV antibody index- > 0.9, and for 21 of 25 patients (84%), all samples had an anti-JCV antibody index > 1.5 (Fig 5). In 1 patient, 3 of 4 available samples had an anti-JCV antibody index- 0.9, 2 of which were collected within 12 months of PML diagnosis."
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