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Diagnostic Tests of Diabetes Mellitus

Diabetes Mellitus is categorized by disorders of carbohydrate, protein and fat metabolism. Diagnostic tests are numerous, most of them are given below

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Urine “Glucose”

–   lacks sensitivity     = positivity in disease

–   poor specificity             = negativity in health

•    Problems

–   renal threshold variable 6 to 15 mmol/L

–   interferences : Clinitest / Glucose oxidase strips

If urine test positive a confirmatory  blood test is needed

Blood Tests

•    Glucose

–   whole blood 10-15% lower than plasma

–   venous 10% lower than capillary

–   Preservatives not 100% effective

•  heparin / serum – loss of 1 mmol/L per 2 hours

•  fluoride / oxalate – loss of about 10% overnight

•    Insulin / glycosylated Hb / fructosamine

–   No use for diagnosis

Diagnostic Criteria

•    Plasma glucose :

A) Symptoms + random value > 11.1 mmol/L

B) Fasting value > 7.0 mmol/L

C) 2-h post glucose load (75g) value > 11.1 mmol/L

If one of these criteria is met then repeat testing the following day is required for confirmation

Oral Glucose Tolerance Test (OGTT)

•    Complicated test no longer recommended for routine use except in pregnancy

•    Still available in many laboratories

•    Still found in text books

•    Used in testing in pregnancy

•     This evaluates the endogenous insulin response to a physiological glucose challenge

•     The diagnostic value of OGTT has been over estimated and is used too frequently in situations where it is not needed

•     In the absence of a specific marker of the diabetic state the OGTT remains the most sensitive and practical test for the early recognition of asymptomatic diabetes, without fasting hyperglycaemia even if it is a non-specific test.

Indications of OGTT

•     Evaluation of a person with a borderline elevation of fasting or post prandial plasma glucose

•     Diagnosis of gestational D.M

•     High risk individuals

•     Evaluation of glycosuria

•     Assessment of unexplained presence of what might be expected a diabetic complication e.g. Neuropahty

•     Metabolic condition (elevated triglycerides)

•     Obesity

•     Clinical or epidemiological research projects

Preparation Of Patient For OGTT

•      The patient must be ambulatory and free from pyrexia, acute illness or trauma for at least two weeks.

•      He/she should have diet containing at least 150 g carbohyrate per day for three days prior to test

•      Any drug that alter blood glucose  level should also be discontinued prior to testing for 03 days.

•      The test should be done in the morning

•      The patient must have 8 – 16 h fast.

•      Heavy tea and coffee drinkers should reduce their consumption

•      Smoking should be avoided

•      No physical exercise or activity is allowed

•      Patient should be seated quietly and relaxed for 30 minute for test

Procedure For OGTT

•      Glucose load

–   For adults 75 g glucose

–   For children 1.75 g/kg body weight

–   Glucose load is given in water (25 g/dl)

•      This blood samples are taken at half hourly interval up to 2 hours

•      Urine samples are usually not required for the diagnosis of diabetes mellitus

Interpretation Of OGTT

•     Normal

–   Fasting < 5.5 mmol/l

–   0.5, 1.0, 1.5 hrs < 10 mmol/l

–   2 hrs sample < 7.8 mmol/l

•     Diabetic

–   Fasting > 7.8 mmol/l

–   2 hours or one intermediate

–   Sample > 11.1 mmol/l

•     Impaired OGTT

–   Fasting < 5.5-7.8 mmol/l

–   One intermediate sample    > 11.1 mmol/l

–   2 hrs between 7.8 and 11.1 mmol/l

OGTT In Gestational D.M

•     OGTT is done with a glucose load of 100g

•     Blood samples are taken up to 3 hrs, at least two values must exceed the following

fasting                5.8 mmol/l

1 hr                      10.5 mmol/l

2 hrs                             8.6 mmol/l

3 hrs                             7.8 mmol/l


Fasting Plasma Glucose

•      Reference range for plasma glucose for venous blood sample (3.3-5.5 mmol/l) – 60-100 mg/dl

•      Duration of fast 10-14 hrs

•      It is a specific screening test

•      Glucose value > 7.8 mmol/l (140 mg/dl) is diagnostic

•      Precautions

–    Normal diet (carbohydrate 150g/day)

–    Absence of diseases which are known to cause hyperglycemia

•      Limitations

–    Fasting hyperglycemia occurs relatively late in some patients of NIDDM

•    Post challenge plasma glucose/OGTT is suggested when fasting glucose is between 5.5-7.8 mmol/l.


•    Note: after 50 yrs of age; add 0.6 mmol per decade or 1 mg/year to the diagnostic criteria


Random Plasma Glucose

•      In healthy individuals, plasma glucose conc. Vary only slightly throughout the day and generally are in the range of 2.5-7.3 mmol/l. The only rise that occurs is found following a meal.

•      Diagnostic value

–   > 11.1 mmol/l – dm

–   < 2.5 mmol/l hypoglycaemia 7.8-11.1 mmol/l

–   Further investigations

•      Random glucose has no value in screening, diagnosis and follow up of DM

•      It has greatest value in comatosed patients, whether known or preveiously unknown diabetes.

Post Challenge Plasma Glucose

•      Glucose load = 75 g

•      Plasma glucose conc. Is determined 2 hrs later

•      It allows for more objective conditions of testing and a more precise comparison of an individual’s response with a large population. This test is more precise than a postraprandial glucose

•      However this test requires more preparation

•      Unrestricted activity

•      Carbohydrate diet

•      Fasting

•      Diagnostic value – 11.1 mmol/l

Two Hour Postprandial Plasma Glucose

It has been used to screen for diabetes mell-itus, to diagnose diabetes and to monitor glucose control. The significance of the 2 hrs postprandial value is limited by the lack of rigidly controlled conditions

•      Amount of carbohydrate

•      Time used to consume meals

•      Type of food

•      Age of the patient

•      Time of the day

These factors are responsible for the differences in the interpretation of the results

•     Interpretation:

–   Glucose value < 7.8 mmol/l    normal

–   Glucose value  7.8 – 11.1 mmol/l   ogtt

–   Glucose value >11.1 mmol/l dm

Sampling For Blood Glucose

A. Whole blood allowed to stand at room temperature shows decrease in glucose from 7-10% in one hour

•      The erythrocytes of newborn infants consume glucose more rapidly (20-30%/hrs)

•      Procedures for the prevention of glycolysis in blood specimens should be adopted to prevent decrease in glucose before estimation

B. Venous/capillary blood

C. Plasma/whole blood

•      It provides more stable values for glucose

•      No effect of hematocrit

•      It reflects the extracellular glucose concentration more accurately


•      Fasting plasma glucose can be offered as a single screening test

•      The patients having fasting glucose level between 5.5 – 7.8 mmol/ can be further evaluated with either post challenge glucose or OGTT

•      Random glucose should only be reserved for patient presenting as medical emergency

•      1 mg/dl per year of age (0.6 mmol/decade)after the age of 50 years must be added to the accepted normal range of glucose for correct interpretation of the results.

Looking for complications

Need to monitor for long-term complications – biochemistry is useful as a screen

–   Renal

•   Plasma creatinine (50-140 µmol/l)

•   Urine “microalbumin”

•   < 30 µg/min in resting overnight sample

–   Lipids

•   Fasting triglycerides (< 1.7 mmol/l)

•   Cholesterol (< 5.1 mmol/l)

–   ? Thyroid function tests

•   Autoimmune association IDDM

•   May be justified especially in elderly females

Acute events

Diabetic Ketoacidosis

•    Caused by insulin deficiency and often precipitated by intercurrent illness

•   increased glucagon, cortisol, catecholamines, GH  causes increased hepatic glycogenolysis & gluconeogenesis  and unrestrained FFA release from adipose tissue

•   beta-oxidation in liver produces excess ketone bodies causing acidosis

•   dehydration due to osmotic diuresis leads to renal impairment and failure to compensate for acidosis

•    Usually a long prodromal period followed by a rapid onset of acidosis and catastrophic collapse

DKA – Biochemistry

•    Severe Acidaemia & Acidosis

–   pH may be 6.75 – 6.9 (ref 7.35 – 7.45)

–   Kussmaul breathing

•    Hyperglycaemia

•    Hyperkalaemia

–   H+ exchange across membranes for K+

–   but note this masks a severe intracellular K+ deficiency

DKA -diagnostic testing

•    Clinician – at bedside

–   test strip blood glucose

–   arterial blood pH, pO2, pCO2

•   Laboratory

–   confirms glucose

–   plasma U&E (creatinine unreliable)

•  low sodium

–   may be due to glucose osmotic dilution

–   may be pseudo due to hyperlipidaemia

•  high potassium

–   due to acidosis

Hypoglycemic Coma

Symptoms : coma, tachycardia, sweating, fits

•    Immediately test strip glucose

–   beware credibility of low results

–   and failure to diagnose

•    Treat if in any doubt – glucose is harmless!

•    Confirm with lab blood/plasma glucose BUT take sample before treatment

–   blood glucose < 2.2 mmol/L

–   plasma glucose < 2.5 mmol/L

Hyperosmolar Hyperglycaemic coma (HONK)

•    Elderly NIDDM

–   some mild ketosis, lack of hyperventilation

–   50% raised plasma Na > 150mmol/L

–   glucose typically 50 – 60 mmol/L

•    50% mortality – widespread venous thrombosis

•    Good prognosis if well managed

Want a clearer concept, also see

Diabetes Mellitus
Endocrine Functions of Pancreas

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  1. Thanks for the work well presented. My questions and uncertainities got cleared by the straight forward and well organised information. My academic challenge questions found an applicable quide.

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