1. Several methods of quantifying muscle capillary basement membrane width have been evaluated with the aim of determining whether variations in technic may influence the sensitivity of detecting significant capillary basement membrane thickening in diabetic subjects.

2. The results demonstrate that fixation of tissue in osmic acid followed by determination of mean basement membrane thickness reveals significant basement membrane thickening in over 90 per cent of diabetic subjects.

3. The error in estimation of mean basement membrane width due to oblique sectioning of diabetic capillaries is shown to be small, i.e. approximately 10 per cent. By contrast, measuring only the minimum basement membrane thickness of a capillary may underestimate diabetic basement membrane hypertrophy by from 20 to 40 per cent.

4. As compared to osmic acid fixation, glutaraldehyde leads to a very significant increase in the basement membrane width affecting normal capillaries relatively more than diabetic capillaries.

5. As a result, either fixation in glutaraldehyde or measurement of minimum basement membrane thickness may lead to a marked decrease in the sensitivity of the quantitative basement membrane technic. A procedure that employs both glutaraldehyde fixation and measurement of only minimum basement membrane width would appear to detect microangiopathy in only 45 per cent of diabetic subjects.

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