Glycosuria: Mechanism and Evaluation
GLYCOSURIA has, until recently, been thought of in terms of a “renal threshold” by which glucose is retained or excreted according to its concentration in blood. Useful as it has been, this explanation is physiologically unsound (Govaerts and Muller,1 Mirsky and Nelson).2 Furthermore it does not resolve clinical problems to which newer concepts of glycosuria apply directly. Hence, the clinician has often had to lead his glycosuric patient through a maze of glucose tolerance tests, measurements of arterial (capillary) and venous blood sugar, and even changes of diet, only to emerge with an insecure conclusion.
The purpose of this paper is to describe the mechanism of glycosuria as it applies to clinical problems, and to suggest a procedure which may aid in establishing the nature of glycosuria in doubtful cases.
Mechanism of Glycosuria
Formation of urine begins with the outpouring through glomerular capillaries of an ultrafiltrate of plasma. About 1200 cc. of blood, containing 650 cc. of plasma, pass through the kidneys and over the glomerular capillary bed per minute. The hydrostatic pressure furnished by the heart squeezes out from the 650 cc. of plasma about 130 cc. of filtrate. This filtrate consists of plasma water and of the substances dissolved in it. Among these is glucose. Consequently (fig. 1) at a plasma glucose concentration of 100 mg. per 100 cc., the rate of glucose filtration is 130 mg. per minute.
The next step in urine formation is selective reabsorption. This consists in a transfer from the tubule back into. . .