I would be glad to assist you.
There has been evidence that ranitidine can cause false positive drug
screening tests for many years. The screening tests use an immunoassay method for testing, so is based on monoclonal antibodies to the drug.
However, there have been reports that ranitidine can cause false positive results. For example, see http://www.clinchem.org/content/36/7/1391.2.full.pdf+html
This was a letter to the editor about a report in an earlier edition of a single case that was falsely positive from ranitidine, but this letter included a greater amount of information, because eight individuals were studied and a false positive result was found in five of eight individuals.
It is important to note that there are many false positives that are never explained, which is why it is generally recommended that all positive results by immunoassay should be confirmed by a better test, such as GC/MS. So, even if you were not taking any ranitidine, it would have been better that a confirmatory test was done.
If a confirmatory test was not done, it is difficult to prove a year later that it was definitely a false positive. The journal articles can certainly show that it is possible, but there is no testing that can be done at this point that will prove it.
If I can provide any further information, please let me know.