Page 844 - Textbook of Pathology, 6th Edition

828 Then 75 gm of glucose dissolved in 300 ml of water is given. 6. Insulin assay. Plasma insulin can be measured by
Blood and urine specimen are collected at half-hourly radioimmunoasay and ELISA technique. Plasma insulin
intervals for at least 2 hours. Blood or plasma glucose content deficiency is crucial for type 1 DM but is not essential for
is measured and urine is tested for glucosuria to determine making the diagnosis of DM.
the approximate renal threshold for glucose. Venous whole 7. Proinsulin assay. Proinsulin is included in immunoassay
blood concentrations are 15% lower than plasma glucose of insulin; normally it is <20% of total insulin.
values. 8. C-peptide assay. C-peptide is released in circulation
Currently accepted criteria for diagnosis of DM (as per
American Diabetes Association, 2007) are given in Table 27.8: during conversion of proinsulin to insulin in equimolar
quantities to insulin; thus its levels correlate with insulin level
Normal cut off value for fasting blood glucose level is in blood except in islet cell tumours and in obesity. This test
considered as 100 mg/dl. is even more sensitive than insulin assay because its levels
Cases with fasting blood glucose value in range of 100- are not affected by insulin therapy.
125 mg/dl are considered as impaired fasting glucose tolerance 9. Islet autoantibodies. Glutamic acid decarboxylase and
(IGT); these cases are at increased risk of developing diabetes islet cell cytoplasmic antibodies may be used as a marker for
later and therefore kept under observation for repeating the type 1 DM.
test. During pregnancy, however, a case of IGT is treated as
a diabetic. 10. Screening for diabetes-associated complications.
Individuals with fasting value of plasma glucose higher Besides making the diagnosis of DM based on the defined
than 126 mg/dl and 2-hour value after 75 gm oral glucose criteria, screening tests are done for DM-associated
complications e.g. microalbuniuria, dyslipidaemia, thyroid
higher than 200 mg/dl are labelled as diabetics (Fig. 27.27,D). dysfunction etc.
In symptomatic case, the random blood glucose value
above 200 mg/dl is diagnosed as diabetes mellitus. ISLET CELL TUMOURS
V. OTHER TESTS. A few other tests are sometimes Islet cell tumours are rare as compared with tumours of the
performed in specific conditions in diabetics and for research exocrine pancreas. Islet cell tumours are generally small and
purposes: may be hormonally inactive or may produce hyperfunction.
1. Glycosylated haemoglobin (HbA1C). Measurement of They may be benign or malignant, single or multiple. They
blood glucose level in diabetics suffers from variation due are named according to their histogenesis such as: β-cell
to dietary intake of the previous day. Long-term objective tumour (insulinoma), G-cell tumour (gastrinoma), A-cell
SECTION III
assessment of degree of glycaemic control is better tumour (glucagonoma) D-cell tumour (somatostatinoma),
monitored by measurement of glycosylated haemoglobin vipoma (diarrhoeagenic tumour from D cells which elaborate
1
(HbA ), a minor haemoglobin component present in normal VIP), pancreatic polypeptide (PP)-secreting tumour, and
1C
persons. This is because the non-enzymatic glycosylation of carcinoid tumour. However, except insulinoma and
haemoglobin takes place over 90-120 days, lifespan of red gastrinoma, all others are extremely rare and require no
blood cells. HbA assay, therefore, gives an estimate of further comments.
1C
diabetic control and compliance for the preceding 3-4
months. This assay has the advantage over traditional blood Insulinoma (ββ ββ β-Cell Tumour)
glucose test that no dietary preparation or fasting is required. Insulinomas or beta (β)-cell tumours are the most common
Increased HbA value almost certainly means DM but islet cell tumours. The neoplastic β-cells secrete insulin into
1C
normal value does not rule out IGT; thus the test is not used the blood stream which remains unaffected by normal
Systemic Pathology
for making the diagnosis of DM. Moreover, since HbA 1C regulatory mechanisms. This results in characteristic attacks
assay has a direct relation between poor control and of hypolgycaemia with blood glucose level falling to 50 mg/
development of complications, it is also a good measure of dl or below, high plasma insulin level (hyperinsulinism) and
prediction of microvascular complications. Care must be high insulin-glucose ratio. The central nervous mani-
taken in iterpretation of the HbA value because it varies festations are conspicuous which are promptly relieved by
1C
with the assay method used and is affected by presence of intake of glucose. Besides insulinoma, however, there are
haemoglobinopathies, anaemia, reticulocytosis, transfusions other causes of hypoglycaemia such as: in starvation, partial
and uraemia. gastrectomy, diffuse liver disease, hypopituitarism and
hypofunction of adrenal cortex.
2. Glycated albumin. This is used to monitor degree of
hyperglycaemia during previous 1-2 weeks when HbA can MORPHOLOGIC FEATURES. Grossly, insulinoma is
1C
not be used. usually solitary and well-encapsulated tumour which may
3. Extended GTT. The oral GTT is extended to 3-4 hours for vary in size from 0.5 to 10 cm. Rarely, they are multiple.
appearance of symptoms of hyperglycaemia. It is a useful Microscopically, the tumour is composed of cords and
test in cases of reactive hypoglycaemia of early diabetes. sheets of well-differentiated β-cells which do not differ
4. Intravenous GTT. This test is performed in persons who from normal cells. Electron microscopy reveals typical
have intestinal malabsorption or in postgastrectomy cases. crystalline rectangular granules in the neoplastic cells. It
is extremely difficult to assess the degree of anaplasia to
5. Cortisone-primed GTT. This provocative test is a useful
investigative aid in cases of potential diabetics. distinguish benign from malignant β-cell tumour.