A publication in the NCBI bookshelf called Type 2 diabetes: Measuring sugar levels in blood and urine yourself (IQWiG, 2010) it states that
Having sugar in your urine is usually a sign of very high blood sugar levels. The extra sugar in the bloodstream is usually only removed via the kidneys at blood sugar concentrations of 10 mmol/L (180 mg/dL) and above. In order to measure the amount of sugar in your urine, you need a urine test strip and a container for collecting urine.
To find out what the results mean, the colors on the test strip are compared with the color chart on the package. If the colors on the test strip don’t change, you don’t have sugar in your urine. The more the color changes, the more sugar there is in the urine, and therefore in the blood.
In the answer from @Jan to the question Why does kidney disease result in some things being filtered out but not others? it is pointed out that
healthy kidneys do not excrete glucose or proteins in significant amounts.
and mentions glycosuria which is a rare condition in which the kidneys release glucose into the urine
Renal glycosuria can cause urine glucose levels to be high even if blood glucose levels are normal (Stang & Reed-Guy, 2018).
Seeing as the book on the NCBI bookshelf is talking about urine glucose levels related to type 2 diabetes, (and also talks of type 1 diabetes testing using urine), is urine testing that reliable for checking for high blood glucose levels or the possibility of diabetes in a patient when glucose is generally only released when there is kidney disease?
IQWiG. (2010). Type 2 diabetes: Measuring sugar levels in blood and urine yourself. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG). Retrieved from NCBI Bookshelf: https://www.ncbi.nlm.nih.gov/books/NBK279508/
Stang, D. & Reed-Guy, L. (2018). Urine Glucose Test. Healthline. Retrieved from: https://www.healthline.com/health/glucose-test-urine