Hi Uriel,
Have you considered Discriminant Analysis (Analyze > Multivariate Methods > Discriminant analysis)? If you have not, here are the basic steps I would implement:
1) Using your clearly diagnosed patients (Hide and Exclude questionable patients), select Diagnosis (or whatever you call their status) as X Categories and your variables as Y, Covariates
2) Run the analysis and under the main pull-down menu (red triangle) select Score Options > Save Formula (Several Calculation columns will be appended at the end of your table)
3) Check your Hidden/Excluded row for which the the probability of Diagnosis is automatically calculated
Now, this method assumes that there is a true diagnosis in your "reference" patients which might not be correct. Hence, you can explore the effect of including ALL patients in your discriminant analysis which will not assume that some patients are more important than others
Best,
TS
Thierry R. Sornasse