How can you have sugar in urine but low blood sugar?

Low threshhold: Ordinarily the kidney does not spill sugar into the urine until the blood level reaches about 180. However, if this kidney threshhold happens to be set at a much lower level, the sugar will begin to spill at a much lower level. This goes by the scientific name of renal glycosuria and has no relation to diabetes.

Timing: Urine is collected over hours in the bladder. If a patient with diabetes has sugar in the urine that means that blood sugar control from over a few to several hours back was likely high at some point. However, it doesn't say anything for the current situation where blood sugar can certainly be low.