a yr ago my vl showed less than 50,recently it is less than 100.does it means the exact numbers cannot be counted.is the difference significant

There are several different types of commercial viral load assays, and they have different lower thresholds, e.g. < 50 copies/ml, <100 copies/ml, < 400 copies/ml, etc. In your specific case, it sounds as though the two tests were performed with different tests. You should consider these two test results to be the same.

What each test means exactly is that the assay cannot reliable provide information about viral load levels below their threshold, e.g. < 50 copies/ml. In studies using 'ultra-sensitive' assays, the exact numbers have been defined, and they do indeed range between 1 to 49 copies/ml, and very few are actually 0.

I urge you to talk to your doctor about your question and this response.