Albuminuria in Diabetes Mellitus
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p405-406
2025-11-03
55
Urinary albumin is an essential biomarker for the development of diabetic nephropathy and is also a known marker of endothelial damage, which leads to a significant risk of cardiovascular disease. Therefore, its determination is indicated in diabetic patients at risk of renal (renal failure) and cardio vascular complications.
The progression of urinary albumin excretion increases the likelihood of developing chronic renal failure. In addition, the presence of subclinical amounts of albumin in the urine (previously identified by the term “microalbuminuria,” to be dropped) is considered a cardiovascular risk factor for diabetic and nondiabetic patients.
Even after the institution of the treatment plan, measurement of albuminuria is recommended to evaluate the effectiveness of treatment. Guidelines indicate that urine albumin measurement should be performed annually in patients with type 1 diabetes with a disease duration greater than 5 years, all patients with type 2 diabetes regardless of disease duration, and pregnant patients. For the diagnosis of diabetic nephropathy, albuminuria must be positive in at least two out of three samples collected within 3–6 months.
In the DCCT and UKPDS trials, it was found that intensive therapy for diabetes reduces the risk of increased urinary albumin excretion (UAE) and overt nephropathy in diabetic patients. A new guideline on managing chronic kidney dis ease, which also covers the use of albuminuria measurement, has recently been published and is referred to for further discussion.
Preanalytical Aspects
The standardization of urinary albumin measurement is not optimal, so making clear and consistent recommendations is essential. Urinary albumin excretion can be increased by physiological factors such as exercise, posture, and diuresis, and samples should not be collected after strenuous exercise (up to 2–3 days before testing), in the presence of a urinary infection, during a period of convalescence, or after surgery. The urine sample from the 24-h collection and the first- morning specimen (on which to measure albumin and creatinine) are suitable. The collection of the first-morning sample is preferable because it is less affected by biological variability than the other methods listed.
Due to the high individual and daily variability, performing at least three different urine collections on three different days is advisable. The urine sample should then be stored at 4 °C (stable for up to 2 weeks) or at −80 °C if more extended storage is required (stable for up to 5 months). Freezing at −20 °C is not recommended. Freezing the sample may lead to a decrease in albumin. To avoid this problem, the sample should be thoroughly mixed before analysis. Visually turbid samples should be centrifuged to remove precipitates and cellular components. Possible bacterial contamination and glucose do not affect the investigation.
Analytical Aspects
Both semi-quantitative and quantitative tests are available for the determination of increased UAE. Semi-quantitative tests are recommended only in screening programs because they have low sensitivity since they cannot detect modest increases in urinary albumin excretion, which characterize the early stages of diabetic nephropathy. In contrast, quantitative assays are highly sensitive.
Several methods have been developed for measuring albumin in urine, including radioimmunoassay, enzyme- linked immunosorbent assay (ELISA), radial immunodiffusion, immunoturbidimetry, and nephelometry. Each method has advantages and disadvantages, so the choice depends on the laboratory. In general, however, these assays have similar inaccuracies, limitations, and reference ranges. In any case, the positivity for urinary proteins with the test strips must be confirmed by laboratory measurements. Regarding the measurement method for albumin, the recommendations for immunochemical techniques are a coefficient of variation (CV) <15% and a detection limit of approximately 2 mg/L. Based on observational clinical studies, other indications suggest a more stringent value for imprecision: CV <13 % for albumin and CV <6 % for the albumin/creatinine ratio.
Postanalytical Aspects
The reference intervals for making a diagnosis of diabetic nephropathy differ by type of sample collection and are shown in Table 1.
Table1. Reference ranges of urinary albumin
Units can be in mg/24 h for 24-h excretion or in mg/g creatinine, which gives the same result. The use of mg/mmol creatinine generates a different numerical value. The suggestion for deciding which unit to use is to establish uniformity of reporting among laboratories in the same geographic area. It is strongly discouraged to report albumin in mg/L.
There are no different decision levels for the sex of the diabetic patient, while for the albuminuria/cardiovascular risk association, the decision levels are still under study, but epidemiological studies suggest a lower concentration than that decided for diabetic nephropathy.
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