Sodium tolbutamide (1 g is given IV in 20 mL of saline

over 2 minutes to a (12 hours) fasting subject. A fasting

preinjection blood specimen and postinjection samples

at 2, 5, 10, 20, 30, 60, 120, and 180 minutes are collected

for glucose estimation. Juvenile diabetics (insulinopenic)

reveal virtually no response, while adult (maturity-onset)

diabetics show a delayed increase in blood glucose concentrations. Patients with an insulin-secreting tumor (islet cell

adenoma or hyperplasia) reveal a profound depression

of blood glucose values, which persists below 50 mg% at

2 hours, this is associated with maximum insulin values

as early as 15 minutes. Appropriate medical precautionary measures must be readily available (sterile glucose

injection) and used promptly with any stress tolerance test

whenever a patient’s condition warrants intervention and

cessation of test.

IV Insulin Tolerance Test

Here insulin 0.1 unit/kg of ideal body weight is

administered IV in a fasting state, blood specimens are

collected at appropriate intervals over a 2 hour period

for glucose analysis. Within 30 minutes the blood glucose

concentration falls to about 50 or 60% of the fasting level

and returns to normal fasting levels between 1 hour

30 minutes, and 2 hours. A failure to observe such a

depression in blood glucose concentration may imply

insulin resistance. This may be occasionally seen in adult

type diabetes, as well as in acromegaly and Cushing’s

syndrome. In panhypopituitarism and adrenocortical

insufficiency (Addison’s disease) a more profound and

sustained decrease in blood glucose may be observed,

hence caution should be exercised in patients suspected

of having these disorders.

Glycosylated Hemoglobin (HbA1c); Glycohemoglobin

(GHb); Diabetic Control Index

Kits Available Commercially

Increased

Diabetes mellitus, glycosuria, and hyperglycemia.

Decreased

See, below, Factors that affect results.

Description

Glycosylated hemoglobin is blood glucose bound to

hemoglobin (Hb) and includes from HbA1a, HbA1b, and

HbA1c. HbA1c is formed as hemoglobin, is gradually

glycosylated throughout the 120 days; red blood cell

lifespan, and forms the largest portion of the three

Diabetes Mellitus: Laboratory Diagnosis 443

glycosylated Hb fractions. The amount of glycosylated

hemoglobin found and stored in erythrocytes depends on

the amount of glucose available. HbA1c is a reflection of

how well blood glucose levels have been controlled for up

to the prior 4 months. Hyperglycemia in diabetics if usually

a cause of an increase in HbA1c.

Factors that Affect Results

a. Reject hemolyzed specimens.

b. Falsely increased values may be due to fetal-maternal

transfusion, hemodialysis, hereditary persistence of

fetal hemoglobin, neonates and pregnancy.

c. Falsely decreased values may be due to anemia

(hemolytic, pernicious, sickle cell); chronic loss of

blood ; effects of splenectomy; renal failure (chronic);

and thalassemias.

Other Data

a. Glycosylated hemoglobin cannot be used to monitor

control of diabetic clients with chronic renal failure,

as levels are significantly lower due to shortened

erythrocyte survival.

Approximately 8.5% of total hemoglobin: HbA1

Glycohemoglobin is one of the types of minor

hemoglobins found in every individual. Hemoglobin A,

undergoes change or glycosylation to hemoglobin A1a, A1b,

A1c by a slow, nonenzyme process within the RBCs during

their circulating lifespan of 120 days. Simply putting it,

glycohemoglobin is blood glucose bound to hemoglobin.

The RBC, as it circulates, combines, some of the glucose

from the bloodstream with its own content of hemoglobin

to form glycohemoglobin in a one-way reaction. The

amount of glycosylated hemoglobin found and stored

by the RBC depends on the amount of glucose available

to it over the RBCs 120 days lifespan. In diabetics with

hyperglycemia, the increase in GHb is usually caused

by an increase in HbA1c. The glucose concentration will

increase when hyperglycemia caused by insulin deficiency

develops. This glycosylation is irreversible.

Test Significance

This test is an index of long-term glucose control. GHb

monitoring reflects the average blood sugar level for the

2 to 3 months period before the test. The more glucose

the RBC is exposed to, the higher the percentage of GHb.

The test provides vital information about the success of

treatment of diabetes such as the adequacy of dietary

or insulin therapy, allows determination of duration of

hyperglycemia in new cases of juvenile onset diabetes with

acute ketoacidosis, provides a sensitive estimate of glucose

imbalance in mild cases of diabetes, and is an evaluation of

effectiveness of old and new forms of therapy such as oral

hypoglycemic agents, single or multiple insulin injections,

and B-cell transplantation. Test results are not affected

by time of day, meal intake, exercise, just administered

diabetic drugs, emotional stress, patient cooperation or

accuracy.

The estimation of GHb is of greater importance

for specific groups of patients. These groups include

diabetic children, diabetics in whom the renal threshold

for glucose is abnormal, unstable insulin-dependent

diabetics in whom blood sugars vary markedly from day

to day, patients who do not test urine regularly for glucose,

and people who, before their scheduled appointments,

will change their usual habits, dietary or otherwise, so that

their metabolic control appears better than it actually is.

Clinical Relevance

1. Values are increased in poorly controlled and newly

diagnosed diabetes. In these instances, HbA1c levels

comprise 8 to 12% of the total hemoglobin.

2. With optimal insulin control, the HbA1c levels return

toward normal.

3. A diabetic patient who has only recently come under

good control may still have a high concentration of

glycosylated hemoglobin. This level will only gradually

decline as newly formed RBCs with nearly normal GHb

replace older RBCs with high concentrations of GHb.

Interfering Factors

1. Spurious results should be expected in every case of

hemoglobinopathy distinguishable from hemoglobin

A by electrophoresis.

2. Decreased value in pregnancy and sickle cell anemia,

increased value in thalassemia.

Confusion in interpretation of results may occur

because there are two tests for determining glycosylated

hemoglobin. The most specific test measures HbA1, which

includes hemoglobin A1a, A1b and A1c. There are different

expected values for each test. Keep in mind that HbA1, is

always 2% to 4% higher than HbA1c.

GLYCOSYLATED HEMOGLOBIN KIT (ION

EXCHANGE RESIN METHOD) FOR THE

QUANTITATIVE DETERMINATION OF

GLYCOHEMOGLOBIN IN BLOOD (FOR IN VITRO

DIAGNOSTIC USE ONLY)

(Courtesy: Tulip Group of Companies)

Summary

Glycosylated hemoglobin (GHb) is formed continuously

by the adduction of glucose by covalent bonding to the

444 Concise Book of Medical Laboratory Technology: Methods and Interpretations aminoterminal valine of the hemoglobin beta chain

progressively and irreversibly over a period of time and

is stable till the life of the RBC. This process is slow, nonenzymatic and is dependent on the average blood glucose

concentration over a period of time.

A single glucose determination reflects the glucose

level at the time. GHb on the other hand reflects the mean

glucose level over an extended period of time. Thus GHb

reflects the metabolic control of glucose level over a period

of time unaffected by diet, insulin, other drugs, or exercise

on the day of testing. GHb is now widely recognized as an

important test for the diagnosis of diabetes mellitus and is

a reliable indicator of the efficacy of therapy.

Principle

Glycosylated hemoglobin (GHb) has been defined

operationally as the fast fraction hemoglobins HbA1

(Hb A1a, A1b, A1c) which elute first during column

chromatography. The nonglycosylated hemoglobin, which

consists of the bulk of hemoglobin, has been designated

HbA0.

 

Impact of the New Diagnostic Criteria

Physicians may be concerned that the new diagnostic

criteria for diabetes mellitus, including the lower cutoff

for fasting plasma glucose levels, may greatly increase

the number of people who are diagnosed with diabetes

mellitus in their practices.

Concerns about overdiagnosis include the harm created

by anxiety, the risks and costs of unnecessary treatment,

and possible insurance discrimination, especially if the

condition that is being diagnosed is relatively benign or if

no effective treatment is available.

On the other hand, underdiagnosing a condition is

harmful if early treatment can make a difference in patient

outcome, especially if the treatment is relatively benign

and inexpensive.

It is true that a rigorous screening program will increase

the number of persons who are diagnosed with diabetes

mellitus. However, currently only half of the people who

have diabetes mellitus according to the old criteria have

not been diagnosed and may remain undiagnosed for up

to 10 years.

People who are asymptomatic and underdiagnosed

continue to develop the complications of diabetes mellitus.

Screening Recommendations

The expert committee provided guidelines governing

the selection of patients to be tested for diabetes and the

frequency of that testing (Table 17.3). Testing should be

considered for all persons who are 45 years of older and

should be repeated at 3 years intervals.

Testing should be considered at a younger age and

be performed more frequently in persons who are

obese (120% of desirable body weight or greater or a

body mass index of 27 kg per m2

 or greater); who have

a first-degree relative with diabetes mellitus; who have

delivered a baby weighing more than 4.032 g (9 lb), or

who were diagnosed with gestational diabetes mellitus

during pregnancy; are hypertensive; or have a highdensity lipoprotein level of 35 mg per dL (0.90 mmol per

L) or lower and/or a trigly-ceride level of 250 mg per dL

(2.83 mmol per L) or higher. In addition, any patient with

440 Concise Book of Medical Laboratory Technology: Methods and Interpretations impaired glucohomeostasis should be reevaluated on a

more frequent basis.

The expert committee recommended that screening for

gestational diabetes mellitus should be reserved for use

in women who meet one or more of the following criteria:

25 years of age older, obese (defined as more than 120%

above their desirable body weight), a family history of a

first-degree relative with diabetes mellitus, and belong to

high-risk ethnic population.

Final Comment

The changes recommended by the expert committee for

the diagnosis of diabetes mellitus should prove beneficial

to patients. Measurement of fasting plasma glucose levels

should be more acceptable to the patients than the oral

glucose tolerance test and can be readily incorporated with

fasting lipid determinations. Identifying asymptomatic

persons earlier in the disease process will allow earlier

institution of lifestyle changes and medical therapy that

may decrease the complications of hyperglycemia. The

National Diabetes Data Group (US) emphasizes that

these changes in diagnostic criteria have not changed the

treatment goals in patients with diabetes mellitus. These

goals include maintaining a fasting plasma glucose level of

less than 120 mg per dL (6.65 mmol per L) and a glucose

hemoglobin measurement of less than 7.0%.

Conventional Diagnostic Tests

Oral Glucose Tolerance Test (OGTT)

This is performed to establish a diagnosis in:

1. Patients with transient or sustained glycosuria who

have no clinical symptoms of diabetes (polyuria) and

with normal fasting and post-prandial blood glucose

levels.

2. Patients with symptoms of diabetes but with no

glycosuria and normal fasting level.

3. Persons with a strong family history of diabetes but

with no overt diabetes.

4. Patients whose glycosuria is associated with pregnancy,

thyrotoxicosis, liver disease, and/or infections.

5. Women who have characteristically large babies (> 9

lbs) or individuals who were large babies.

6. Patients with neuropathies and retinopathies of

undetermined origin.

The patient should ingest a daily diet of atleast 300 g

of carbohydrate for 3 days prior to the test. Therefore, on

an acutely ill-hospitalized patient, this test should not

be conducted. As far as possible it should be performed

on an ambulatory patient. Preferably, the test should be

performed in the morning. Various malignancies, fever,

cachexia, liver dysfunction and renal failure may be

associated with mild to moderate degrees of abnormal

GTT. There is an age-related factor that decreases glucose

tolerance and hence makes the interpretation of OGTT in

elderly subjects difficult. Timing of glucose administration

and blood sampling must be accurate.

Patient Preparation

1. Instruct the patient about the purpose and procedure

of the test:

a. Stress a normal diet with high carbohydrate

(150–300 g) for 3 days preceding the test.

b. Fasting is required for at least 10 hours before the

test and not more than 16 hours.

c. Water is permitted and encouraged.

2. Determine the patient’s weight and record it.

3. Collect urine and blood samples and test for glucose,

recording exact time of collection. Have the patient

empty his or her bladder for each specimen:

a. No liquids other than water can be taken. Have

the patient empty his or her bladder for each

urine sample.

b. No food is to be taken during the test period.

c. No alcohol to be consumed the previous evening.

TABLE 17.3: Recommendations for diabetes screening of

asymptomatic persons

Timing of the first test and repeat tests

• Test at age 45:

Repeat every three years (patients 45 years of age or older)

• Test before age 45:

Repeat more frequently than every three years if patient has

one or more of the following risk factors:

a. Obesity ≥ 120% of desirable body weight or BMI ≥ 27 kg

per m2

b. First-degree relative with diabetes mellitus

c. Member of high risk-ethnic group (Black, Hispanic,

Native American, Asian)

d. History of gestational diabetes mellitus or delivering a baby

weighing more than 4.032 g (9 lb)

e. Hypertensive (≥ 140/90 mm Hg)

f. HDL cholesterol level ≥ 35 mg per dL (0.90 mmol per L)

and/or triglyceride level ≥ 250 mg per dL (2.83 mmol per

L)

g. History of IGT or IFG on prior testing

BMI = body mass index: HDL = high density lipoprotein;

IGT = impaired glucose tolerance; IFG = impaired fasting glucose

Diabetes Mellitus: Laboratory Diagnosis 441

d. Encourage the patient to stay in bed or rest quietly during the test period. Weakness or feeling

faint may occur during test, and exercise also

changes, glucose results.

e. No smoking is allowed during the test.

f. Coffee and unusual physical exercise should be

avoided for at least 8 hours before the test.

Carbohydrate meal (or glucose) to be given in 25%

(w/v) solution according to the age.

 Age Dose

 0–18 months 2.5 g/kg

 1½–8 years 2.0 g/kg

 8–12 years 1.75 g/kg

 > 12 years 1.25 g/kg

Preferably, the samples of urine and whole blood be

taken at fasting, 30 minutes, 1, 1½, 2, 3, and 4 hours after

ingestion of the carbohydrate meal. If nausea and vomiting

occur during the test, the interpretation becomes difficult.

Interpretation

Three popular methods for evaluating GTT for diabetes

mellitus are:

1. Wilkerson point system

Time mg% plasma Points

Fasting 130 or more 1

1 h 195 or more ½

2 h 140 or more ½

3 h 130 or more 1

Two or more points are judged diagnostic of diabetes

mellitus (DM).

2. The Fajans-Conn criteria

Time mg% plasma

Fasting

1 h 195 or more

1½ h 165 or more

2 h 140 or more

A diagnosis of DM in otherwise healthy and ambulatory

individuals under age 50 is made if the above criteria are

met.

3. The university group diabetes mellitus program

The fasting 1 h, 2 h and 3 h blood glucose levels are

adjusted for plasma glucose as above, and the subject is

judged diabetic if the sum of values obtained equals 500

or more.

Abnormally, high values in the first hour with a rapid

fall to normal values or a flat curve with no appreciable rise

usually reflect primary alterations in intestinal absorption

of glucose. The former is characteristic of hyperthyroidism

and the latter of hypothyroidism or malabsorptive states.

A very flat rise in blood glucose followed by a prolonged

and pronounced hypoglycemic phase may be observed in

primary (islet cell adenoma or hyperplasia) and secondary

hyperinsulinism (hypoadrenocorticism). In the elderly,

especially in females, interpretation of OGTT must be

made in light of what is an age-dependent carbohydrate

intolerance.

Interfering Factors

1. Smoking will increase glucose level.

2. Inadequate diet (such as a weight-reducing diet)

before testing can diminish carbohydrate tolerance

and suggest a false diabetes.

3. Levels tend to increase normally in older people, the

maximum can reach 200 mg/dL.

4. Prolonged administration of oral contraceptives will

give significantly higher glucose levels in the second

hour or in later blood samples.

5. Bedrest over a lengthy period of time will influence

glucose tolerance. For this reason, the test should be

performed on ambulatory patients, not on patients

whose condition requires bedrest.

6. Infectious diseases and surgery will affect tolerance.

Two weeks of recovery should be permitted before the

test.

7. Drugs that impair glucose tolerance:

• Insulin

• Oral hypoglycemics

• Salicylates in larger doses

• Oral contraceptives

• Thiazide diuretics

• Corticosteroids

• Estrogen

• Ferrous ascorbinate

• Nicotinic acid

• Phenothiazines

• Lithium

• Metapyrone.

Discontinue these drugs for 3 days prior to test. This

test is contraindicated in patients who have had a recent

history of surgery, myocardial infarction, a labor and

delivery, for these conditions can cause erroneous results.

Record and report any reactions that may occur during

the test. Weakness, faintness, and sweating may occur

between the second and third hours. If this occurs, a blood

sample for sugar is drawn and the test is discontinued. Test

should be postponed in the event of unexpected illness,

such as fever or gastritis or if there has been ingestion

of food within 8 hours. If the fasting blood sugar is over

200 mg/dL, the GTT is usually not done. If it is done, the

patient should be monitored very carefully for severe

reaction or even coma.

442 Concise Book of Medical Laboratory Technology: Methods and Interpretations IV GTT

In patients with gastrointestinal disorders an intravenous

GTT may be done. These patients may be suffering from

sprue or malabsorption syndrome or may be postgastrectomy patients. A sterile glucose solution is given IV

(20% w/v) over a 30 minutes period in an amount of 0.5 g/

kg of ideal body weight. Similar blood collection intervals,

including a fasting specimen, are followed, and a curve is

plotted for evaluation (F, 1/2, 1, 1½, 2, 2½ and 3 hours).

In a normal individual, the fasting specimen of blood

contains a normal amount of glucose, the concentration

of any single specimen does not exceed 250 mg%, and

by 1 hour 30 minutes to 2 hours, the blood glucose

level approximately comes to the fasting level (this test,

however, is a less sensitive indicator of mild abnormalities

of carbohydrate tolerance than the standard OGTT).

Rapid IV GTT

Here a rapid IV (50% w/v) GTT (0.5 g glucose/kg ideal body

weight) to a maximum dose of 25 g may be given over a 3-4

minute period. Blood samples are obtained at intervals of

10 minutes for at least 2 hours. Under these conditions,

disappearance of glucose from blood follows an exponential

curve and a glucose disappearance constant can be

calculated. In normal subjects, glucose disappearance

usually exceeds 1.5% of the administered dose per minute,

values below 1% are compatible with diabetes mellitus.

Cortisone Glucose Tolerance Test

This may reveal prediabetic patients, especially in relatives

of known diabetics. Cortisone promotes intolerance in a

latent or mild diabetic. After performance of an initial GTT,

a standard dose of cortisone (50 mg) for adults is given

parenterally 8½ hours and again 2 hours before a regular

GTT. A positive test shows a blood glucose concentration

of 140 mg% or higher with 2 hours specimen. Follow-up

studies are necessary for such individuals.

Parenteral Administration of Glucagon or

Epinephrine

Will cause a slight elevation of blood glucose concentration

from glycogenolysis in normal subjects—this is much

greater and more sustained in diabetics. It is also a measure

of glycogen storage and release, so it may be used to study

patients suspected of having a glycogen storage disease.

IV Tolbutamide Test

Oral hypoglycemic agent administration results in

secretion of insulin from pancreas. This principle is taken

advantage to indicate (active) insulin reserve in patients.

 

The definition and diagnosis of gestational diabetes

mellitus was not altered in these new recommendations.

Gestational diabetes mellitus is an operational

classification (rather than a pathophysiologic condition)

identifying in women who develop diabetes mellitus

during gestation (Women with diabetes mellitus before

pregnancy are said to have “pregestational diabetes” and

are not included in this group). Women who develop

type 1 diabetes mellitus during pregnancy and women

with undiagnosed asymptomatic type 2 diabetes mellitus

that is discovered during pregnancy, are classified with

gestational diabetes mellitus. However, most women,

classified with gestational diabetes mellitus, have normal

glucose homeostasis during the first half of the pregnancy

and develop a relative insulin deficiency during the last

half of the pregnancy, leading to hyperglycemia. The

hyperglycemia resolves in most women after delivery but

places them at increased risk of developing type 2 diabetes

mellitus later in life.

New Diagnostic Criteria for Diabetes Mellitus

The new diagnostic criteria for diabetes mellitus have been

greatly simplified in Table 17.2.

The oral glucose tolerance test previously recommended

by the National (American) Diabetes Data Group has been

replaced with the recommendation that the diagnosis of

diabetes mellitus be based on two fasting plasma glucose

levels of 126 mg per dL (7.0 mmol per L) or higher.

Type 1 diabetes mellitus*

Type 2 diabetes mellitus*

Other specific types:

Genetic defects of beta-cell function

Genetic defects in insulin action

Diseases of the exocrine pancreas

Pancreatitis

Trauma/pancreatectomy

Neoplasia

Cystic fibrosis

Hemochromatosis

Others

Endocrinopathies

Acromegaly

Cushing’s syndrome

Glucagonoma

Pheochromocytoma

Hyperthyroidism

Somatostatinoma

Aldosteronoma

Others

Drug-or chemical-induced

Vacor

Pentamidline

Nicotinic acid

Glucocorticoids

Thyroid hormone

Diazoxide

Beta-adrenergic antagonists

Thiazides

Phenytoin

Infections

Congenital rubella

Cytomegalovirus

Others

Uncommon forms of immune-mediated diabetes

Other genetic syndromes sometimes associated with diabetes

Down syndrome

Klinefelter’s syndrome

Turner’s syndrome

Wolfram syndrome

Friedreich’s ataxia

Huntington’s chorea

Laurence-Moon Beidel syndrome

Myotonic dystrophy

Porphyria

Prader-Willi syndrome

Others

Gestational diabetes mellitus

Contd...

Contd...

438 Concise Book of Medical Laboratory Technology: Methods and Interpretations Other options for diagnosis include two 2-hour

postprandial plasma glucose (2 h PPG) readings of 200 mg

per dL (11.1 mmol per L) or higher after a glucose load of

75 g (essentially, the criterion recommended by WHO) or

two casual glucose readings of 200 mg per dL (11.1 mmol

per L) or higher. Measurement of the fasting plasma glucose

level is the preferred diagnostic test, but any combination

of two abnormal test results can be used. Fasting plasma

glucose was selected as the primary diagnostic test because

it predicts adverse outcomes (e.g. retinopathy) as well as

the 2 h PPG test but is much more reproducible than the

oral glucose tolerance test or the 2 h PPG test and easier to

perform in a clinical setting.

The choice of the new cutoff point for fasting plasma

glucose levels is based on strong evidence from a number

of population linking the risk of various complications to

the glycemic status of the patient. As per the study on the

risk of diabetic retinopathy based on the glycemic status of

40 to 74 years old participants in the National Health and

Nutritional Epidemiologic Survey (NHANES III) the risk

of retinopathy greatly increases when the patient’s fasting

plasma glucose level is higher than 109 to 116 mg per dL

(6.05 to 6.45 mmol per L) or when the result of a 2 h PPG

test is higher than 150 to 180 mg per dL (8.3 to 10.0 mmol

per L) or when the result of a 2 hr PPG test is higher than

150 to 180 mg per dL (8.3 to 10.0 mmol per L). However,

the committee decided to maintain the cutoff point for the

2 h PPG test at 200 mg per dL (11.1 mmol per L) because

so much literature has already been published using this

criterion. They selected a cutoff point for fasting plasma

glucose of 126 mg per dL (7.0 mmol per L) or higher. This

point corresponded best with the 2 h PPG level of 200 mg

per dL (11.1 mmol per L). The risk of other complications

also increases dramatically at the same cutoff points.

A normal fasting plasma glucose level is less than

110 mg per dL (6.1 mmol per L) and normal 2 h PPG

levels are less than 140 mg per dL (7.75 mmol per L).

Blood glucose levels above the normal level but below

the criterion established for diabetes mellitus indicate

impaired glucose homeostasis. Persons with fasting

plasma glucose levels ranging from 110 to 126 mg per dL

(6.1 to 7.0 mmol per L) are said to have impaired fasting

glucose, while those with a 2 h PPG level between 140 mg

per dL (7.75 mmol per L) and 200 mg per dL (11.1 mmol

per L) are said to have impaired glucose tolerance. Both

impaired fasting glucose and impaired glucose tolerance

are associated with an increased risk of developing type

2 diabetes mellitus. Lifestyle changes, such as weight

loss and exercise, are warranted in these patients. The

committee chose not to address the current controversies

surrounding the diagnosis of gestational diabetes mellitus

and did not alter the diagnostic criteria in this area.

Screening for gestational diabetes mellitus is generally

accomplished with administration of a 50 g glucose load

one hour before determining a plasma glucose level.

A positive screen [defined as a plasma glucose level of

140 mg per dL (7.75 mmol per L) or higher] should prompt

a diagnostic test; fasting plasma glucose levels should be

measured after a 100 g glucose load at baseline and at

1, 2 and 3 hours after the glucose load. Two of the four

values must be abnormal [105 mg per  dL (5.8 mmol per

L) or higher; 190 mg per dL (10.5 mmol per L) or higher;

165 mg per dL (9.15  mmol per L) or higher; and 145 mg

per dL (8.05 mmol per L) or higher) for a patient to be

diagnosed with gestational diabetes mellitus. The WHO

criteria use a glucose load of 75 g with a test two hours after

the glucose load, using the same criterion for the diagnosis

of gestational diabetes mellitus.

Glycated Hemoglobin

Measurements of glycated hemoglobin have commonly

been used to monitor the glycemic control of persons

already diagnosed with diabetes mellitus. Measurements

of this hemoglobin, also called glycosylated hemoglobin,

TABLE 17.2: Criteria for the diagnosis of diabetes mellitus and

impaired glucose homeostasis

Diabetes Mellitus

Positive findings from any two of the following tests on different

days:

a. Symptoms of diabetes mellitus* plus casual † plasma

glucose concentration ≥ 200 mg per dL (11.1 mmol per L)

or

b. FPG ≥ 126 mg per dL (7.0 mmol per L)

or

c. 2 h PPG ≥ 200 mg per dL (11.1 mmol per L after a 75 g

glucose load)

Impaired Glucose Homeostasis

a. Impaired fasting glucose:

FPG from 110 < 126 (6.1 to 7.0 mmol per L)

b. Impaired glucose tolerance:

2 h PPG from 140 to < 200 (7.75 to < 11.1 mmol per L)

c. Normal

FPG < 110 mg per dL (6.1 mmol per L)

2 h PPG < 140 mg per dL (7.75 mmol per L)

†

Casual is defined as any time of day without regard to time since last meal

*Symptoms include polyuria, polydipsia or unexplained weight loss. FPG =

fasting plasma glucose; 2 hr PPG = two-hour postprandial glucose

Diabetes Mellitus: Laboratory Diagnosis 439

glycohemoglobin, hemoglobin A1c or hemoglobin A1, aid

in the evaluation of the stable linkage of glucose to minor

hemoglobin components. There is currently no agreement

on standardization, so a variety of measurement methods

and normal ranges are being used.

Some experts argue that a glycated hemoglobin test

could be used for the diagnosis of diabetes mellitus.

Glycated hemoglobin levels are as highly correlated to

adverse clinical outcomes (e.g. retinopathy) as are fasting

plasma glucose or postprandial plasma glucose levels and

are as reproducible as fasting plasma glucose levels. The

major advantage of measuring glycated hemoglobin is that

the specimen can be collected without regard to when the

patient last ate.

The expert committee, however, did not include glycated

hemoglobin measurement in the recommendations for

international standards for the diagnosis of diabetes

mellitus. They noted the lack of standardization and

normal ranges among the various tests, making it difficult

to dictate a standard cutoff point.

The test for measuring glycated hemoglobin is not

widely available in developing countries; consequently, it

was not favored for use as an international criterion. There

is also some overlap in the levels of glycated hemoglobin

in patients with diabetes mellitus and those without it.

Although, it was not specifically recommended by the

National Diabetes Data Group (US) as a diagnostic test for

diabetes mellitus, glycated hemoglobin may, in some case,

be used to diagnose diabetes mellitus.

The diagnosis of diabetes mellitus is made in the

following fashion. A glycated hemoglobin level of 1%

above the reference laboratory’s upper range of normal is

consistent with diabetes mellitus and has a specificity of

98%. People with normal glycated hemoglobin levels (i.e.

within the laboratory’s published normal range) either do

not have diabetes mellitus or have well-controlled diabetes

mellitus (i.e. a false-negative test). However, incorrectly

diagnosing these persons as normal would not alter

their treatment because exercise and diet are adequately

controlling their blood glucose levels.

People who are not diagnosed with diabetes mellitus

and who have near-normal glycated hemoglobin levels

(less than 1% above the normal range) may be advised of

the high probability that they have diabetes mellitus and

may be offered the same treatment as a person with mild

diabetes mellitus (i.e. dietary and exercise counseling),

followed by repeat testing of glycated hemoglobin several

months later.

This method of screening and counseling high-risk

persons is easier for many patients and clinicians because

the blood specimen can be drawn at the time of the patient

visit.

Glycated hemoglobin (also known as glycohemoglobin, glycosylated hemoglobin or HbA1c) is used to monitor treatment in patients

with diabetes mellitus; however, it is not recommended for routine

diagnosis of this condition because of a lack of standardization of

tests and results.