Diabetes Mellitus Laboratory findings

1.

Urinalysis

-
a. Glucosuria - A specific and convenient method to detect glucosuria is the paper strip impregnated with glucose oxidase and a chromogen system (Clinistix, Diastix), which is sensitive to as little as 0.1% glucose in urine. Diastix can be directly applied to the urinary stream, and differing color responses of the indicator strip reflect glucose concentration.
A normal renal threshold for glucose as well as reliable bladder emptying is essential for interpretation.

b. Ketonuria - Qualitative detection of ketone bodies can be accomplished by nitroprusside tests (Acetest or Ketostix). Although these tests do not detect β-hydroxybutyric acid, which lacks a ketone group, the semiquantitative estimation of ketonuria thus obtained is nonetheless usually adequate for clinical purposes.

2.

Blood testing procedures

-
a. Glucose tolerance test -
(1) Methodology and normal fasting glucose - Plasma or serum from venous blood samples has the advantage over whole blood of providing values for glucose that are independent of hematocrit and that reflect the glucose concentration to which body tissues are exposed. For these reasons, and because plasma and serum are more readily measured on automated equipment, they are used in most laboratories. If serum is used or if plasma is collected from tubes that lack an agent to block glucose metabolism (such as fluoride), samples should be refrigerated and separated within 1 hour after collection. The glucose concentration is 10-15% higher in plasma or serum than in whole blood because structural components of blood cells are absent.

(2) Criteria for laboratory confirmation of diabetes mellitus - If the fasting plasma glucose level is 126 mg/dL or higher on more than one occasion, further evaluation of the patient with a glucose challenge is unnecessary. However, when fasting plasma glucose is less than 126 mg/dL in suspected cases, a standardized oral glucose tolerance test may be done.
For proper evaluation of the test, the subjects should be normally active and free from acute illness. Medications that may impair glucose tolerance include diuretics, contraceptive drugs, glucocorticoids, niacin, and phenytoin.
Because of difficulties in interpreting oral glucose tolerance tests and the lack of standards related to aging, these tests are being replaced by documentation of fasting hyperglycemia.

b. Glycated hemoglobin (hemoglobin A1) measurements - Hemoglobin becomes glycated by ketoamine reactions between glucose and other sugars and the free amino groups on the α and β chains. Only glycation of the N-terminal valine of the beta chain imparts sufficient negative charge to the hemoglobin molecule to allow separation by charge dependent techniques. These charge separated hemoglobins are collectively referred to as hemoglobin A1 (HbA1). The major form of HbA1 is hemoglobin A1c (HbA1c) where glucose is the carbohydrate. HbA1c comprises 4-6% of total hemoglobin A1. The remaining HbA1 species contain fructose-1,6 diphosphate (HbA1a1); glucose-6-phosphate (HbA1a2); and unknown carbohydrate moiety (HbA1b). The hemoglobin A1c fraction is abnormally elevated in diabetic persons with chronic hyperglycemia. Methods for measuring HbA1c include electrophoresis, cation-exchange chromatography, boronate affinity chromatography, and immunoassays. Office-based immunoassays using capillary blood give a result in about 9 minutes and this allows for immediate feedback to the patients regarding their glycemic control.Since glycohemoglobins circulate within red blood cells whose life span lasts up to 120 days, they generally reflect the state of glycemia over the preceding 8-12 weeks, thereby providing an improved method of assessing diabetic control. The HbA1c value, however, is weighted to more recent glucose levels (previous month) and this explains why significant changes in HbA1c are observed with short-term (1 month) changes in mean plasma glucose levels. Measurements should be made in patients with either type of diabetes mellitus at 3- to 4-month intervals so that adjustments in therapy can be made if HbA1c is either subnormal or if it is more than 2% above the upper limits of normal for a particular laboratory. In patients monitoring their own blood glucose levels, HbA1c values provide a valuable check on the accuracy of monitoring. In patients who do not monitor their own blood glucose levels, HbA1c values are essential for adjusting therapy. Data from the Diabetes Control and Complications Trial (DCCT) showed that there is a linear relationship between the HbA1c and the mean of seven-point capillary blood glucose profiles (preprandial, postprandial, and bedtime). Thus, mean plasma glucose levels of 170, 205, 240, and 275 mg/dL approximately correlate with HbA1c values of 7%, 8%, 9%, and 10%, respectively. Use of HbA1c for screening is controversial. Sensitivity in detecting known diabetes cases by HbA1c measurements is only 85%, indicating that diabetes cannot be excluded by a normal value. On the other hand, elevated HbA1c assays are fairly specific (91%) in identifying the presence of diabetes.The accuracy of HbA1c values can be affected by hemoglobin variants or derivatives; the effect depends on the specific hemoglobin variant or derivative and the specific assay used. Immunoassays that use an antibody to the glycated amino terminus of β globin do not recognize the terminus of the γ globin of hemoglobin F. Thus, in patients with high levels of hemoglobin F, immunoassays give falsely low values of HbA1c. Cation-exchange chromatography separates hemoglobin species by charge differences. Hemoglobin variants that co-elute with HbA1c can lead to an overestimation of the HbA1c value. Chemically modified derivatives of hemoglobin such as carbamoylation (in renal failure) or acetylation (high-dose aspirin therapy) can similarly co-elute with HbA1c by some assay methods.Any condition that shortens erythrocyte survival or decreases mean erythrocyte age (eg, recovery from acute blood loss, hemolytic anemia) will falsely lower HbA1c irrespective of the assay method used. Alternative methods such as fructosamine should be considered for these patients. Vitamins C and E are reported to falsely lower test results possibly by inhibiting glycation of hemoglobin.