How to improve OCR (Scanning) results?"
Build a dictionary of common words that frequently appear in these documents (e.g., MEDICAL, SEX, AGE, etc).  Then, for each word in the output from OCR, check whether it is similar to a word in your dictionary; if so, then replace it with the dictionary word.  "Similar" might be defined as "edit distance &lt;= 1".

For example, your sample output has the word "sax".  If your dictionary contains the word "SEX", "sax" would be detected as a misspelling of "SEX" (edit distance = 1).

This post-processing will help you detect some of the common words and help you identify the fields.  What it won't do is clean up errors in the OCR of codes, as those can be anything.  There's probably no good way to handle that.

Also, do a Google search on how to use Tesseract.  There are some best practices that seem to improve its output (e.g., convert to greyscale TIFF format, deskew text, binarize, and more).

Finally, some commercial OCR software is significantly better than Tesseract or any other free OCR.  e.g., Abbyy seems to be well-regarded.