The high risk group who must be tested for diabetes
It is always wise to subject a patient for testing of blood sugar when he has some features which put him into the high risk group. Hence apart from symptomatic patients, blood sugar testing to rule out diabetes must be done in those with a family history of diabetes, obesity (BMI over 27 in men/over 25 in women) especially those with central obesity (W/H ratio over 0.9 for men and over 0.8 for women), hyperlipidaemia, non healing ulcers, pulmonary tuberculosis at atypical sites, recurrent infections, premature macrovascular disease, women with bad obstetrical history, persons taking diabetogenic drugs and all persons over 40.
Role of urine sugar:
Urine sugar must not be used to diagnose diabetes for the first time.
Benedict's test has the advantage of being cheap but also has the disadvantage of being less sensitive, less specific, less convenient.
Although the strips are specific and convenient it has the disadvantage in the form of the colour changes being difficult to distinguish, as also inhibition of colours by ketones.
Apart from the above mentioned problems there are other limitations for urine glucose testing as e.g. hyperglycemia can occur without glucose in urine, urine glucose level may not correspond to prevailing blood glucose levels. Negative test does not distinguish between hypoglycemia, euglycaemia, or mild hyperglycemia and urine glucose tests do not give warning of impending hypoglycemia. Hence urine glucose monitoring should be supplemented with periodic blood glucose estimation.
Blood glucose estimation
Blood glucose estimation using the enzymatic method is the ideal method (Glucose oxidase method). Blood glucose can be estimated as fasting blood glucose, 2 hours post prandial blood glucose and random blood glucose. But it also has its limitations as it tells about the state of control just at the moment of measurement and also occasional blood glucose measurement does not allow accurate assessment of the diabetic state, Different principles and methods in use for measurement of blood glucose are as follows
Principle
|
Substance measured
|
Problems & interference |
| a) Oxidation reduction Somogy-nelson Folin & Wu |
Reducing substance |
Haemolysis Glycoysis protein Precipitation |
|
b) Calorimelric
O-TuIuidine
|
Aldohexose
|
Strum bilirubin
Serum Lipids
|
|
c) Enzymatic GOD Hexokmase
Glucose dehvdrogenase
|
Glucose
|
Bilirubin Uric acid
Ascorbic acid
|
|
d) Physical methods
Mass spectrometry optical rotation Radio-isotopic assay Infra red spectroscope'
|
Glucose
|
Haemolvsis Glucolysb
Not in use.
|
The blood glucose estimation can be affected by many factors. The host factors which can affect blood glucose are diet, weight, age, sex, physical activity, illness-trauma, emotional status, endocrinopathies, pregnancy, drugs. The technical factors responsible are, the time of day, type of test, size of glucose load, sampling time, site of taking blood samples, type of blood samples, preservation of samples, method of analysis.
Dry chemistry of glucose measurement
Various kinds of strips and reflectance meters are in use for measurement of blood glucose such as dextrostrix, glucostix, haemoglucostix. They are glucose oxidase method. There are some important points about their use which should be kept in mind. The strips measure plasma glucose and not whole blood glucose, and being capillary the results are 10 to 20% higher than venous whole blood in post prandial state. Accurate timing and good compliance with operational instructions are essential as improper use can lead to errors upto 20 mgm%. Over exposure and under exposure of blood to the strip will over estimate or underestimate blood sugar reading. Too much of washing may remove blood from the strip. Excessive humidity may discolour the strip and such strips may give low values. Anaemic patients give elevated values. Dextrostrix reads from 40 to 400 mgm% and haemoglucostix reads from 20 to 800 mgm%.
Diagnostic criterion - for diabetes mellitus:
Fasting venous plasma glucose 140 mgrn% on at least two separate occasions and 200 mgm% after 2 hrs. post prandial.
Glucose tolerance test
A. Preparation
The patient should be on (Normal) diet for 48-72 hours with 300 gms. of carbohydrates per day. The patient should be fasting for 8-10 hours starting from previous night and blood is collected in the fasting state. The glucose load for OGTT is 75 gms. for nonpregnant adults and 100 gms. for pregnant women and 1.75 g/kg. body weight in children (max.75gms). The glucose solution is taken within 2-3 minutes. The patient should be at rest during the test and smoking being strictly prohibited. Blood is collected at intervals of 1/2,1,1-1/2 and 2 hours and later on every 60 min. for 3 hrs. in pregnant women.
B, OGTT - WHO criterion of diagnosis of diabetes mellitus
The sample used is venous whole blood
| |
Fasting
|
2 hours P.P. |
Impression |
Remarks |
Glucose value
|
<100 mgm% |
<120 mgm% |
Normal
|
Retest annually if indicated |
<120 mgm%
|
120 to 179 mgm% |
Impaired GTT |
Retest annually |
| <120 mgm% |
<1SO mgm% |
Diabetes Mellitus
|
Treat |
C. Impaired glucose tolerance test
Patients with impaired G.T.T may develop overt diabetes mellitus with time and they may also develop other complications such as hypertension, hyperuricaemia, lipid disorders and obesity and they also have more chance of developing coronary artery disease.
Optimal glycaemic control is achieved when the blood glucose level remains in the euglycaemic range during the whole 24 hours period.
Self monitoring of blood glucose (SMBG)
SMBG is indicated in those patients with-multiple insulin therapy, widely fluctuating blood glucose (brittle diabetes), unawareness of hypoglycemia, in pregnancy and in acute illness where tight control is necessary, with abnormal renal threshold and in the perioperative period.
The advantages of SMBG is that it is convenient and rapid, there is increased flexibility to patient's life style and there is early recognition of hyper and hypoglycemia.
SMBG is handicapped by the fact that it is costly and if proper techniques ai'e not used then it may give erroneous results.
Parameters to be judged in long term glucose control:
Glycated haemoglobin gives an idea about glucose control over a period of 8-12 weeks. Serum fructosamine level give an idea about glucose control over a period of 2 weeks and is ideal in gestational diabetes. Maintenance of ideal weight and B.P. control is also important as is the diagnosis of long term complications in their initial stages viz. renal, retinal, peripheral vascular, cardiovascular, neurological and dyslipidaemias.
Glycated haemoglobin (Hb AIC)
Hb AIC gives an idea about long term control of blood glucose over a period of 8-12 weeks. Chromatography and thiobarbitone calorimetric techniques is used for its measurement. It should be kept in mind that inter laboratory variations are high cJue to standardization problem. It may be falsely elevated with fructose rich diet, hyperlipidaemia, uraemia, elevated temperature and elevated pH of blood and false low values may be seen in pregnancy, anaemia, after blood transfusion, low temp, and low pH of blood.
Serum fructosamine
Serum fructosamine assesses control over a period of 2-4 weeks, but is influenced by hyper bilirubinaemia and hyperlipidaemia. It can be used to diagnose stress diabetes and gestational diabetes mellitus.
Assessment of diabetic control
For NIDDM assessment, fasting and post prandial blood glucose must be done at regular intervals and a proper weight record must also be maintained. Glycated Hb must be measured twice a year. For IDDM in addition to regular fasting and post prandial blood glucose and weight record glycated Hb must be measured at least thrice a year and home blood glucose monitoring must be actively encouraged.
Diabetic control can be said to be good if FBG is 100 mgm% and PPBG 140 mgm% and is fair if FBG is 110 mgm% and PPBG 160 mgm% but is poor if the values are above those mentioned above.
Comparative sensitivity of different blood glucose test
Single value may be diagnostic if it is high enough viz FBG200 mgm%, 2 hrs, PPBG350 mgm%. The 2 hrs. PPBG is more sensitive but FBG is less sensitive though more specific OGTT is even sensitive, even more than IVGTT.
Fallacies to be ruled out when a known diabetic comes with a high blood glucose report
We have to enquire as to whether the patient had missed the oral hypoglycaemic tablet the previous night giving rise to raised fasting blood glucose and omitting the premeal tablet can raise the PPBG. It is to be enquired as to whether the patient had early morning or nocturnal hypoglycaemia. A detailed history of drugs should be taken to exclude those drugs which aggravate the diabetic status viz. corticosteroids and thiazide diuretics. A proper diet history must also be taken.
Conclusion
First time diagnosis of diabetes mellitus must be made by blood glucose measurement on more than two occasions. Urine glucose estimation may be used in adult diabetics for day to day assessment keeping in view its limitations. Glycated Hb estimation must be done once in 4 months if possible. Clinical assessment of chronic complications like metabolic, renal, cardiac, autonomic and visual functions in a diabetic is to be assessed in order to give a global care to the patient. It should be remembered that the treatment of diabetes is beyond hyperglycaemia controlling.
|
