Study Guide:

With respect to skills:

Level 1 = should have confidence in performing the task and can recognize normal signs;

Level 2 = should have performed the task;

Level 3 = should have observed the task performed in real life or on video.

With respect to knowledge:

Level 1 = should understand the subject matter and can apply it to practice;

Level 2 = should have a sound understanding of the subject matter;

Level 3 = should be aware of the importance of the subject matter;

Blood glucose (blood sugar) assay

Please take note before you read on: It is common practice to use the terms “blood glucose” and “blood sugar” interchangeably. Furthermore, when you send a sample of venous blood to the laboratory for “blood glucose” or “blood sugar” assay, the laboratory actually measures plasma concentration and not blood concentration. Plasma values are 10 to 15 % higher than those of whole blood. Nevertheless, most doctors use the term “blood glucose” when in fact it should have been “ plasma glucose”. On the other hand, when you test blood sugar at the bedside using a glucose meter, you are truly measuring whole blood glucose concentration.

• This topic is dedicated to the measurement of blood glucose (blood sugar) value (level). You should review your notes on blood glucose homeostasis and diabetes mellitus.
• According to the World Health Organization (WHO), there are currently 120 to 140 million people with diabetes mellitus worldwide and this figure will double by 2025.
• In Hong Kong, as many as 12% of the Chinese population are victims. Although immunoassay of blood insulin level is possible, blood glucose assay remains the cornerstone in the diagnosis of the disease. An abnormally high blood glucose value under certain controlled conditions is considered diagnostic.
• Blood glucose value measurement can take several formats: Oral Glucose Tolerance Test (OGTT), fasting blood sugar, 2 hours post-prandial blood sugar, or random blood sugar.

The Oral Glucose Tolerance Test (OGTT)

The OGTT is a provocative test in which the patient is given a large dose of glucose to challenge the body’s homeostatic mechanism and ability to regulate blood glucose level within normal limits. You should have learned in other parts of the curriculum that many internal factors can affect blood glucose values, in addition to insulin. If all these other factors are normal, then it can be assumed that the subsequent rise and fall in blood glucose values after the consumption of a glucose load is due mainly to insulin secretion by the pancreatic islet cells in response to hyperglycemia.

Exercise these precautions before and during OGTT.

• Patient should be normally active and not bed-bound before the test.
• Patient should be free of an infectious or other acute illness, not even a common cold.
• Patient should have an adequate carbohydrate diet (at least 150 g/day) for 3 days before the test.
• Patient should be fasting for at least 10 hours but not longer than 16 hours before the test.
• Test should be done in the morning.
• Patient should refrain from smoking or drinking coffee on the morning of the test.
• Patient should refrain from physical activities during the test.
  

Test protocol

1. Sample venous blood for assay of fasting plasma glucose at time 0.
2. Give 75 g of glucose or dextrose dissolved in 200 ml of water orally to the test subject who should drink the solution within 5 minutes.
3. Sample venous blood for plasma glucose assay at 1 and 2 hours.
   

The normal response

• The fasting plasma glucose value is between 3.9 and 6.1 mmol/L (70 and 110 mg/dl).
• The 2-hour plasma glucose value is less than 7.8 mmol/L (140 mg/dl).
  

Impaired glucose tolerance

• A subject’s response is said to be impaired and he is at risk of becoming a victim of diabetes in the future if his 2-hour plasma glucose value is less than 11.1 mmol/L (200 mg/dl) but higher than 7.8 mmol/L (140 mg/dl).
  

Impaired fasting glycemia

When the fasting plasma glucose value is between 6.1 and 7.0 mmol/L (110 and 126 mg/dl), the patient is said to have impaired fasting glycemia. He or she is also at risk of developing diabetes in the future.

The diabetic response

• The fasting plasma glucose value is above 7.0 mmol/L (126 mg/dl).
• The 2-hour plasma glucose value is higher than 11.1 mmol/L (200 mg/dl) even when the fasting value is normal.
  

Factors that can influence the OGTT and yield a diabetic type response:

• Chronic debilitating illnesses;
• Endocrine disease like acromegaly (excessive growth hormone), thyrtoxicosis (excessive thyroid hormone), Cushing’s syndrome (excessive adrenocortical hormone), pheochromocytoma (excessive adrenomedullary hormones or catecholamines);
• Starvation or simply diet low in carbohydrate content;
• Inactivity;
• Obesity;
• Fever;
• Stress, including acute myocardial infarction, trauma, burns and similar conditions;
• Pregnancy;
• Advanced age;
• Many medications, including oral contraceptives, steroids, diuretics, and anticonvulsants;
• Diurnal variation: Result of test done in the afternoon takes on a diabetic profile while that of a test done in the morning is normal.
  

Fasting blood glucose (fasting blood sugar)

Fasting blood glucose (fasting blood sugar) is determined by sampling venous blood after an overnight fast (no caloric intake after midnight). Normal plasma value is between 3.9 and 6.1 mmol/L (70 and 110 mg/dl). A value above 7.0 mmol/L (126 mg/dl) on more than one occasion is an accepted diagnostic criterion for diabetes mellitus.

2-hour post-prandial blood glucose (2-hour post-prandial blood sugar)

Much emphasis is placed on the 2-hour plasma glucose value in the OGTT as a diagnostic criterion for diabetes mellitus. Therefore, plasma glucose value 2 hours after a meal (post-prandial), although less controlled, is commonly used to screen for potential victims of the disease. Elevated 2-hour post-prandial plasma glucose value is an indication for more definitive investigation.

Random blood glucose (random blood sugar)

As its name implies, random blood glucose is plasma value from venous blood sampled at no fixed time of the day. The condition under which a random blood sample is obtained is not controlled. While elevated random blood glucose value is an indication for further investigation, normal random blood sugar level does not rule out diabetes mellitus. By and large, 2-hour post-prandial blood glucose value is a better screening test for diabetes mellitus.

Self-monitoring of blood glucose values

In order to keep blood glucose level under control, a doctor may recommend self-monitoring of blood glucose at various times of the day while the patient is at home or at work so that he can prescribe treatment accordingly. There are a large number of low-cost portable (a fraction of the size of a mobile phone) glucose meters in the marketplace. These devices assay whole blood glucose concentration. The most common technology used is based on a test strip of paper impregnated with an enzyme (usually glucose oxidase) and a color agent (chromogen). When a drop of blood is placed on a prescribed area of the strip, enzymatic oxidation of glucose in the blood is coupled with a change in the color of the chromogen. The intensity of the color change is proportional to the amount of glucose in the drop of blood. This color change is read by a color photometer built into the device, which then displays a numerical value representative of the concentration of glucose in the drop of blood. A newer technology senses the tiny electrical charges generated by the enzymatic oxidation of glucose. Again the amount of electrical charges is proportional to the amount of glucose in the drop of blood and the meter displays blood glucose values proportional to these charges.

Test your own blood glucose

Exercise

1. Read the instructions that accompany the glucose meter supplied to you.
2. Wash your hands with soap and water or clean the tip of a finger of your non-dominant hand with alcohol and wipe dry.
3. Prick the tip of the finger with the spring-loaded lancet.
4. Point your hand downwards and milk your finger in the direction of its tip but do not squeeze. (Squeezing may express tissue fluid that has a different glucose concentration from blood.)
5. Wipe off the first drop of blood with dry gauze or tissue. When the second of blood appears, drop it on to the prescribed area of the test strip. (Wiping off the first drop of blood is recommended to improve accuracy because it may be contaminated with sweat.)
6. Follow the instructions that come with the instrument to obtain your own blood glucose value.
   
   

Bedside chemical urine tests

Chemical tests on urine can be done at the bedside or at home using reagent strips or dipsticks. The number of tests that can be done on a single strip varies from 1 to 10. The following products are available form Bayer Diagnostics Limited:

Attention to details on the storage and handling of these test strips is required to ensure reliability of the tests. Heat or cold, moisture, sun or fluorescent light, acids or alkalis, and volatile fumes can cause the reactivity of the strips to deteriorate. It is important to store them together with the desiccant provided in the original container at room temperature of less than 30oC. They should not be stored in the refrigerator either.

Test your own urine (for glucose, ketone, blood, protein, & pH)

Exercise

1. Bring back a morning sample of your own urine in the container provided.
2. Read the instructions on the insert that accompanies the bottle containing the reagent strips. Sometimes the instructions are also printed on the bottle.
3. Uncap the bottle and remove a strip, taking care not to touch any of the test areas with your fingers. Recap the bottle immediately.
4. Mix the urine to be tested by inverting the container several times.
5. Uncap the container and submerge all the test areas of the strip in the urine sample and remove it quickly so as to avoid dissolving reactive reagents off the strip.
6. While removing the strip, run the edge of its entire length against the rim of the urine container to remove excess urine. Then hold the strip in a horizontal position to avoid possible mixing of chemicals from adjacent reagent areas.
7. Use the minute hand of a watch or clock and time each test according to the manufacturer’s directions: 30 seconds for glucose; 40 seconds for ketone; and 60 seconds for blood, protein and pH.
8. At the specified time for each test, compare the color of the test area with the corresponding color chart on the bottle label and record your results.
   
   

Test a sample of the urine provided

Exercise

1. Bring back a morning sample of your own urine in the container provided.
2. Read the instructions on the insert that accompanies the bottle containing the reagent strips. Sometimes the instructions are also printed on the bottle.
3. Uncap the bottle and remove a strip, taking care not to touch any of the test areas with your fingers. Recap the bottle immediately.
4. Mix the urine to be tested by inverting the container several times.
5. Uncap the container and submerge all the test areas of the strip in the urine sample and remove it quickly so as to avoid dissolving reactive reagents off the strip.
6. While removing the strip, run the edge of its entire length against the rim of the urine container to remove excess urine. Then hold the strip in a horizontal position to avoid possible mixing of chemicals from adjacent reagent areas.
7. Use the minute hand of a watch or clock and time each test according to the manufacturer’s directions: 30 seconds for glucose; 40 seconds for ketone; and 60 seconds for blood, protein and pH.
8. At the specified time for each test, compare the color of the test area with the corresponding color chart on the bottle label and record your results.
   
   

Venous blood sampling

When sending a blood specimen to the laboratory for tests, a request form will have to be filled, blood has to be sampled from the patient by venipuncture, this sample will have to be put into the proper container, and the container must be properly labeled to identify that its content is from the patient.

Preparing the request form and container label

In order to avoid doing the wrong test on the wrong patient, the form used to request the test (request form) and the blood specimen container label must be prepared properly. The request form and the container label must contain the patient’s full name and HKID number. This information must be checked against that on the identification bracelet worn by the patient. If the patient is conscious, it is worthwhile to check the information on the identification bracelet with the patient directly too. In most hospitals, stick-on labels with the patient’s name and HKID number are available for convenience. It is easy in this situation to stick the wrong label on the request form or specimen container. You must develop your own way of checking and crosschecking to ensure that the request form and the container are properly labeled. (N.B. In some hospitals, like the Prince of Wales Hospital, laboratory tests can be ordered through a computerized system without filling out request forms. When you become an intern, you should familiarize yourself with the local system and practice of your hospital. Nevertheless, you still have to workout a way of avoiding mislabeling of specimens.)

Besides the patient’s name and HKID number, most request forms would need additional information:

• Patient’s sex, age, date of birth and location;
• Current illness or a brief history;
• Test(s) requested;
• Your identification code (doctor’s code) and signature.

All should be completed so as to avoid delay.

Blood specimen container

An appropriate volume of blood should be sent in a container designated for the test you have in mind. Some tests required clotted blood; some require anticoagulated blood. The anticoagulant to be used may vary according to the test as well. Such information is available in the laboratory manual of most hospitals. When in doubt, you can always call the laboratory for confirmation.

Sampling blood by venipuncture

• The most common veins from which blood is sampled for laboratory tests are the basilic and cephalic veins at the cubital fossa. At this site, these veins are very superficial and appear just beneath the skin.

Exercise

1. Fill out the request form and prepare the subject’s label. Be sure to double-check information.

2. Gather the necessary equipment: including a 21 G hypodermic needle, a syringe of appropriate volume, venous tourniquet, alcohol, dry gauze swabs, and blood specimen container;

3. Sit the subject down comfortably and explain to him/her the procedure of sampling blood by venipuncture and the potential discomfort from the needle prick.

4. Put on a pair of gloves for protection; this is advisable whenever you are handling body fluids.

5. Rest the subject’s outstretched arm on a flat surface at heart level and turn it supine.

6. Apply the tourniquet around his/her arm; it should be tight enough to cause the superficial veins to standout but not so tight that it occludes the radial pulse.

7. Tap on or rub the skin overlying the vein you want to puncture lightly to make it standout; or if the vein is obscure, you may be able to palpate its course with you index finger.

8. Clean the skin surface overlying the vein with alcohol and leave dry;

9. Hold the syringe between the thumb and the middle, ring and small finger of your dominant hand and steady the needle using your index finger.

With this approach you can change your angle of skin entry at will.

Never hold the syringe like a pencil because it limits your angle of skin entry.

10. Make you skin entry at a low angle along the course of the vein with the bevel of the needle pointing away from the skin.

11. Once you have confirmed entry of the vein by aspiration of blood, advance the needle another short distance into the vein so that its tip is well into the vein lumen and will not slip out.

12. Continue to aspirate blood until the desired volume is withdrawn into the syringe.

13. Once the desired volume of blood is aspirated into the syringe, steady it with one hand and release the tourniquet with the other hand.

14. Use a piece of dry gauze to apply gentle pressure to the entry site while the needle is withdrawn. Ask the patient to continue to apply pressure while you attend to the specimen obtained.

15. Remove the needle from the syringe and drop it into the special box for disposal of sharp instruments immediately.

16. Uncap the specimen container and deposit the blood sample into it directly from the syringe. Injecting blood into the container via the needle is not advisable because: (a) you may inadvertently prick yourself and (b) forcing blood through the needle may cause hemolysis that can adversely affect the analytical fitness of the specimen.

17. Recap the container, attach the subject’s label, and send it off to the laboratory together with the completed request form.