Sunday, July 13, 2008

Renal Threshold For Glucose

You've just been to the doctor. He found sugar in your urine. But your blood sugars are usually in the 90s fasting, and around 140 mg/dl after meals, at least when you check. And isn't the renal threshold, or the level of blood glucose above which kidneys fail to reabsorb, and thus spill glucose into urine, often stated as 180 mg/dl?

Might your sugars be going higher without you knowing it?

In addition to home testing, a simple A1C test might indicate that fact. 1

However, there's the possibility you could be spilling glucose at levels below 180 mg/dl. A fair amount of normal variability exists in the renal threshold - both among individuals and within an individual - which makes urine testing not a very dependable tool for detecting a high blood glucose.

Here are a few references I found to that effect:

Tests Of Glycemia In Diabetes, Diabetes Care, 2004
"Reasons why the use of urine glucose testing to estimate blood glucose concentrations in diabetes management is undesirable include the following:
  1. Although the renal threshold for glucose in healthy adults corresponds to a plasma glucose concentration of ~180 mg/dl (10 mmol/l), there is wide individual variation. Of particular importance are findings that adults, especially those with long-standing diabetes, may have substantial increases in this threshold, resulting in underestimation of the blood glucose level. Conversely, children and, particularly, pregnant women may have very low or variable renal thresholds, resulting in overestimation of the blood glucose level.
  2. Fluid intake and urine concentration affect urine test results.
  3. The urine glucose value reflects an average level of blood glucose during the interval since the last voiding and not the level at the time of the test.
  4. A negative urine glucose test does not distinguish between hypoglycemia, euglycemia, and mild or moderate hyperglycemia. Thus, urine glucose testing is of limited value in preventing hypoglycemia and hyperglycemia.
  5. Urine glucose testing, which uses a color chart with which the test strip color is compared, is less accurate than capillary blood glucose monitoring, which typically uses a digital readout from a reflectance meter.
  6. Some drugs interfere with urine glucose determinations.
  7. Evaluation of urine dipsticks reveals high imprecision at low glucose concentrations. Manufacturers claim that the test strips are positive if urinary glucose concentrations are 100 mg/dl or greater, but the data indicate this does not always occur."
Here's an older study that found pretty wide margins, +/- 150 mg/dl, in estimates of blood glucose from urine glucose...

Validity Of Urine Glucose Measurements For Estimating Plasma Glucose Concentration, Diabetes Care, 1983
"Although our observations show a significant correlation (P less than 0.0001) between plasma glucose concentration and urine glucose concentration or urine glucose excretion rate, the wide confidence limits [95% confidence limits (minimum) +/- 150 mg/dl] on plasma glucose concentration estimated from urine glucose measurements limit the clinical applicability of such estimates."
And one that found a mean of 130 mg/dl, much lower than 180 mg/dl, although it was a special population...

The Influence Of Renal Threshold On The Interpretation Of Urine Tests For Glucose In Diabetic Patients, Diabetes Care, 1980
"In a group of 65 insulin-dependent diabetic patients there was a wide variation in renal threshold, with a mean of 130 mg/dl (range 54-180 mg/dl). Threshold tended to rise with age, and it is suggested that the higher the renal threshold, the higher is the mean blood glucose achieved by the patient."
And a even older study, but telling ...

Renal Threshold For Glucose: Normal And In Diabetics, British Medical Journal, 1940
"The study of the renal threshold in diabetics and normals with healthy renal function shows frequent upward and downward deviations from the accepted "normal" figure of some 170 mg. per 100

The conception of a "normal" threshold is false. There is an average renal threshold, just as there is an average blood pressure, and the many deviations from it should be looked upon as physiological -- a matter of individual idiosyncrasy, and of no pathological significance."

1 HbA1C or just A1C is an abbreviation for glycosylated hemoglobin, blood levels of which can identify average glucose levels for an individual over approximately 120 days, the life span of a red blood cell.