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Role of the Laboratory in Differential Diagnosis of Diabetes Mellitus

Role of the Laboratory in Differential Diagnosis of Diabetes Mellitus. Dr. Essam H. Jiffri. INTRODUCTION. -The demonstration of hyperglycemia or hypoglycemia under specific conditions is used to diagnose diabetes mellitus and hypoglycemic conditions.

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Role of the Laboratory in Differential Diagnosis of Diabetes Mellitus

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  1. Role of the Laboratory in Differential Diagnosisof Diabetes Mellitus Dr. Essam H. Jiffri

  2. INTRODUCTION • -The demonstration of hyperglycemia or hypoglycemia under specific conditions is • used to diagnose diabetes mellitus and hypoglycemic conditions. • -Other laboratory tests have been developed to identify insulinomas and to monitor • glycaemic control and the development of renal complications.

  3. Glucose Estimation • -Glucose may be estimated in either plasma or whole blood. • -The glucose concentration in whole blood is approximately 15% lower than the glucose • concentration in serum or plasma, because the volume of distribution of glucose is lower, as erythrocytes contain less free water than plasma. • -Samples for glucose can be obtained either by veinpuncture or by a fingerprick technique (collected in capillary tubes).

  4. Glucose Estimation • -Blood cells continue to metabolize glucose after veinpuncture and serum or plasma must be refrigerated and separated from the cells within 1 hour to prevent substantial losses of glucose by the cellular fraction. • -A preservative that inhibits glycolysis should be used (sodium fluoride, together with potassium oxalate as an anticoagulant, is used for this purpose).

  5. Glucose Estimation • -Test strips which measure blood glucose can be useful in obtaining an indication of • blood glucose concentrations, but diagnosis should be based on laboratory measurements.

  6. Fasting Plasma Glucose • -A more important measurement is the fasting glucose concentration, which is drawn after an overnight fast (10-16 h). • -A fasting glucose concentration greater than 140 mg/dL (7.8 mmol/L) is considered diagnostic for diabetes mellitus by the National Diabetes Data Group.

  7. Two-Hour Postprandial Plasma Glucose • -The two-hour postprandial glucose measurement is often used in conjunction with the fasting plasma glucose. • -The patient is advised to consume a meal that contains approximately 75 grams of • carbohydrates.

  8. Two-Hour Postprandial Plasma Glucose • -Two hours after eating, a blood sample is drawn for plasma glucose measurement. • -A glucose value greater than 200 mg/ dl (11.1 mmol/L) indicates diabetes mellitus.

  9. Oral Glucose Tolerance Test (OGTT) • -The OGTT is the most sensitive test for the diagnosis of diabetes. • -A sample of the patient's blood is drawn after an over night fast. • -The patient then consumes 75g of a glucose solution and blood is drawn every 30 minutes for two hours.

  10. Oral Glucose Tolerance Test (OGTT) • -For children, glucose is administered at 1.75 9 glucose/kg body weight to a 75 g • maximum. • -A plasma glucose greater than or equal to 200 mg/dL (11.1 mmol/L) at the 2-hour • time point indicates diabetes mellitus.

  11. Oral Glucose Tolerance Test (OGTT) • -Impaired glucose tolerance is diagnosed with a plasma glucose between 140 and 200 mg/dL (7.8 and 11.1 mmo1/L) at 2 hours time point in the test. • -Gestational diabetes is considered present when the values of the OGTT are greater than the following; fasting, 105 mg/dL (5.8 mmo1/L); 1 h, 190 mg/dl (10.6 mmo1/L), • and 2 h, 165 mg/dL (9.2 mmo1/L).

  12. Diagnostic criteria for diabetes mellitus and impaired glucose tolerance

  13. Urinary Glucose • -Glucose can be detected in urine using the specific test strips that contain glucose • oxidase, peroxidase, and a chromagen. • -Other carbohydrates using Benedict's and Febling's reagents.

  14. Urinary Ketones • -Acetone and acetoacetic acid can be detected in urine using the AcetesTM or • KetostixTM systems. • -These tablets or strips use nitroprusside (sodium nitroferricyanide) to detect ketones.

  15. Urinary Ketones • -Because beta-hydroxybutyric acid lacks a ketone group is not detected by this assay. • -Quantitative assays for acetoacetate and beta-hydroxybutyric acid are available using beta-hydroxybutyrate dehydrogenase and either NADH or NAD.

  16. Urinary Ketones • -If NAD is used as the cofactor and the reaction is buffered at around pH 9.0, beta-hydroxyburyric acid is measured. • -On the other hand, a separate reaction using NADH and buffered around pH 7.0 would measure acetoacetic acid.

  17. Glycosylated Proteins and HbA1c • -Long-term blood glucose regulation can be followed by measurement of glycosylated • haemoglobins, this provides the clinician with a time average picture of the patient's • blood glucose concentration.

  18. Glycosylated Proteins and HbA1c • -Many proteins are known • to react with carbohydrates at the peptide N-terminus forming glycosylated peptides. • -Glucose can rapidly • react with hemoglobin • to form a labile • aldimine (Schiff base).

  19. Glycosylated Proteins and HbA1c • -The keto amine product is stable and cannot revert back to hemoglobin and glucose. • -HbA1cis the largest subfraction of normal HbA in both diabetic and non-diabetic • subjects and is formed by the reaction of the-beta chain of HbA With glucose.

  20. Glycosylated Proteins and HbA1c • -The ketoamine (HbA1c) fraction reflects the concentration of glucose present in the • body over a prolonged time period . • -The measurement of glycated haemoglobin therefore gives an indication of the overall • degree of blood glycaemic control, in contrast to glucose measurements which give information for a single time-point.

  21. Microalbuminuria • -Diabetes mellitus causes progressive changes to the kidneys and ultimately results in diabetic renal nephropathy. • -This complication progresses over a period of years and may be delayed by aggressive glycaemic control. • -An early sign that nephropathy is occurring is an increase in urinary albumin.

  22. Microalbuminuria • -Microalbumin measurements are useful to assist in diagnosis at an early stage and • prior to the development of proteinuria. • -Microalbumin concentrations are between 20 to 300 mg/d. • -Proteinuria is typically greater than 0.5 g/d.

  23. Proteinuria in Diabetes • - Many people excrete small quantities of protein in urine, typically around 10 • mg/day of mainly low molecular weight proteins such as albumin. • -Some diabetic patients develop albumin excretion rates 30 µg/min this range • classed as microalbuminuria.

  24. METHODS FOR THE DETERMINATION OF GLUCOSE • The most used • methods of glucose analysis employ the enzymes glucose oxidase or hexokinas. • A) Glucose Oxidase • B) Hexokinase

  25. SELECTED METHODS FOR THE MEASUREMENTS OF GLYCATED HAEMOGLOBINS

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