How are renal glycosuria and diabetes different?

2 different things: For reasons we don't understand, sometimes the kidneys may allow some glucose to slip out into the urine even when the blood level of glucose is normal. This can be detected by a urine dipstick test. In diabetes, the blood level of glucose will be high. In some diabetics, the blood glucose gets so high that the sugar spills over into the urine also.

Whole body vs part: Renal glycosuria only refers to spilling excess sugar from the kidneys into the urine, while diabetes mellitus could be a number of subtypes, and refers to either a partial or total lack of Insulin production, and resistance by the cells of the body to the insulin. There are other parts as well, but this is the main explanation.

Renal tubular defect: Difference: true renal glycosuria occurs without high blood glucose (above 180 mg/dl) due to renal tubular dysfunction. e.g.,Fanconi syndrome, proximal tubular acidosis, or congenital defect of sodium-glucose cotransport enzyme.