Proteinuria in diabetic patients--is it always diabetic nephropathy?

Diabetic nephropathy (dNP) is a consequence of type 1 and type 2 diabetes, typically occurring between 5 and 15 years after diabetes has been diagnosed. The coincidence of dNP and diabetic retinopathy (dRP) is well known. In this study we correlated the histological findings of the kidney biopsy with the clinically expected diagnosis of dNP.

Over a 4-year period with a total of 326 kidney biopsies, 85 biopsies were performed on patients with diabetes. In all of these patients we had information about duration of diabetes and ophthalmological status. Additionally, data about proteinuria, urine sediment and autoantibodies were available. The nephrologist had to give the suspected diagnosis before the biopsy was performed, using the clinical data available.

In 57 patients (67%) dNP was predicted clinically before biopsy. In 28 patients we expected a different kind of kidney disease. Only 43 patients had dNP histologically. In 16 out of 19 patients with dRP we also found dNP. 26 patients with dNP did not have dRP. So dRP was very specific but not sensitive to predict dNP. On the other hand, all patients without dRP but acanthocytes in urine sediment had non-diabetic kidney disease (NDKD). In the case of patients with neither dRP nor acanthocytes, it was very difficult to distinguish between dNP and NDKD. Acanthocytes and antineutrophil cytoplasmatic antibodies with positive antibodies for proteinase 3 or myeloperoxidase were found only in NDKD, but ANAs were detected in a wide titer range in dNP and NDKD. The known duration of the diabetes ranged from 1 to 40 years. There were no additional parameters to differentiate this group.

Diabetic patients with dRP and proteinuria frequently have dNP. In patients without typical retinal findings dNP is less likely, thus a kidney biopsy is necessary to confirm the diagnosis. Additional knowledge about urine sediment and autoantibodies is helpful, but is not sufficient to differentiate NDKD from dNP in the majority of patients.