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Hyperglycemia is diagnosed when fasting plasma glucose exceeds 130 mg/dL or postprandial glucose rises above 180 mg/dL two hours after eating. Additional diagnostic criteria include fasting glucose ≥126 mg/dL, 2-hour oral glucose tolerance test ≥200 mg/dL, random glucose ≥200 mg/dL with symptoms, or HbA1c ≥6.5%. These thresholds help clinicians identify abnormally elevated blood glucose levels.

Insulin promotes glucose uptake into muscle and adipose tissue while suppressing hepatic glucose production. This dual action maintains normal blood glucose levels. When insulin functions properly, it prevents glucose from accumulating in the bloodstream and ensures cells receive adequate glucose for energy production and metabolism.

In type 1 diabetes, autoimmune destruction of pancreatic beta cells causes absolute insulin deficiency, eliminating glucose regulation. Type 2 diabetes involves insulin resistance combined with relative insulin deficiency, impairing glucose control. Both mechanisms result in reduced peripheral glucose uptake and increased hepatic glucose output, causing persistent hyperglycemia despite different underlying pathophysiology.

When plasma glucose exceeds the kidney's reabsorptive capacity around 180 mg/dL, glucose appears in urine, causing osmotic diuresis and polyuria with dehydration, triggering polydipsia. Impaired cellular glucose uptake creates energy deficiency, stimulating polyphagia. Glucosuria also causes calorie loss up to 300 kcal/day, and insulin deficiency promotes fat and muscle breakdown, leading to weight loss.

Missed insulin doses, high carbohydrate intake, illness, infection, stress, and corticosteroid use commonly trigger hyperglycemia. These factors either reduce insulin availability or increase insulin resistance. Management requires medication adherence, dietary control, glucose monitoring, and regular follow-up to prevent episodes and maintain stable blood glucose levels.

Chronic hyperglycemia damages blood vessels, causing microvascular complications such as retinopathy, nephropathy, and neuropathy, plus macrovascular disease including cardiovascular disease and stroke. HbA1c reflects long-term glucose control, while glucose variability also contributes to tissue damage. Understanding these complications of diabetes mellitus helps guide prevention and management strategies.

When HbA1c results are unreliable due to anemia or hemoglobinopathies, alternative diagnostic criteria confirm hyperglycemia: fasting glucose ≥126 mg/dL, 2-hour oral glucose tolerance test ≥200 mg/dL, or random glucose ≥200 mg/dL with classic symptoms. These alternative methods ensure accurate diagnosis when standard HbA1c testing cannot be relied upon.