It certainly is appropriate to check kidney function, but if the only tests that were done were of kidney function, then the next step would be additional blood tests, specifically looking for metabolic conditions that can secondarily affect urine flow.
The first considerations would be the two types of diabetes. The most common type of diabetes, called diabetes mellitus, is due to altered blood sugar metabolism, either because of low levels of insulin or resistance to the effect of insulin. There also can be altered urine flow because of abnormal levels of a different hormone, ADH, which can be high or low, and affect urine flow accordingly. A low ADH causes diabetes insipidus.
I would also usually perform a broader metabolic evaluation, such as for thyroid dysfunction
So, the next step would be additional blood tests including a general chemistry profile, complete blood count, and thyroid profile. It may be necessary to consult with an Endocrinologist, depending upon the initial results, as the evaluation for ADH disorder can be complicated.