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
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.
Genetic defects of beta-cell function
Genetic defects in insulin action
Diseases of the exocrine pancreas
Uncommon forms of immune-mediated diabetes
Other genetic syndromes sometimes associated with diabetes
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.
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
Positive findings from any two of the following tests on different
a. Symptoms of diabetes mellitus* plus casual † plasma
glucose concentration ≥ 200 mg per dL (11.1 mmol per L)
b. FPG ≥ 126 mg per dL (7.0 mmol per L)
c. 2 h PPG ≥ 200 mg per dL (11.1 mmol per L after a 75 g
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)
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
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
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
with diabetes mellitus; however, it is not recommended for routine
diagnosis of this condition because of a lack of standardization of
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