Clinical meaning
Type 2 diabetes mellitus (T2DM) is a progressive metabolic disorder characterized by insulin resistance in peripheral tissues (skeletal muscle, liver, adipose tissue) combined with progressive beta-cell dysfunction leading to relative insulin deficiency. The NP must master the ADA diagnostic criteria and understand the pathophysiological basis for each test. Four diagnostic criteria exist (any ONE is sufficient for diagnosis when confirmed on repeat testing): (1) Hemoglobin A1C ≥6.5%: reflects average glycemia over 2-3 months (the lifespan of a red blood cell); glucose non-enzymatically glycosylates hemoglobin proportional to ambient glucose concentration; advantages include no fasting requirement and low day-to-day variability; HOWEVER, A1C is unreliable in conditions affecting RBC lifespan (hemolytic anemia, sickle cell disease, significant blood loss, iron deficiency anemia, chronic kidney disease, pregnancy) -- these conditions falsely lower A1C by reducing RBC lifespan. (2) Fasting plasma glucose (FPG) ≥126 mg/dL: reflects hepatic glucose output in the fasting state; normally, the liver produces glucose through glycogenolysis and gluconeogenesis to maintain euglycemia during fasting, regulated by the insulin-to-glucagon ratio; in T2DM, hepatic insulin resistance causes excessive hepatic glucose production despite normal or elevated insulin levels; the patient must fast ≥8 hours. (3) 2-hour plasma glucose ≥200 mg/dL during a 75-g oral glucose tolerance test (OGTT): most sensitive test for detecting early glucose intolerance; evaluates both first-phase insulin secretion (rapid insulin release from preformed granules in beta cells, normally peaks at 30 minutes -- this is the FIRST phase lost in T2DM) and peripheral glucose disposal; used primarily for gestational diabetes screening and research. (4) Random plasma glucose ≥200 mg/dL WITH classic symptoms (polyuria, polydipsia, unexplained weight loss): the ONLY test that does not require confirmatory repeat testing. Prediabetes categories (increased risk requiring intervention): A1C 5.7-6.4%, FPG 100-125 mg/dL (impaired fasting glucose, IFG), 2-hour OGTT 140-199 mg/dL (impaired glucose tolerance, IGT). The 'ominous octet' describes eight pathophysiological defects contributing to hyperglycemia in T2DM: (1) decreased insulin secretion (beta-cell failure), (2) increased glucagon secretion (alpha-cell dysfunction), (3) increased hepatic glucose production, (4) decreased muscle glucose uptake, (5) increased lipolysis (adipocyte dysfunction), (6) decreased incretin effect (reduced GLP-1), (7) increased renal glucose reabsorption (SGLT2 upregulation), (8) neurotransmitter dysfunction (brain insulin resistance).