Posted: November 21st, 2014

PERFORMING AN ORAL GLUCOSE TOLERANCE TEST

OGTT.

PERFORMING AN ORAL GLUCOSE TOLERANCE TEST

AIMS
1.    To demonstrate the use of point of care devices in assessing [nutrition related] health.
2.    To develop an understanding of the use of QC to confirm analytical performance.
3.    To develop an understanding of normal and abnormal blood glucose control.
4.    To provide the opportunity for students to further develop data handling, presentation and analytical skills.
5.    To enable students to identify and evaluate hazards, risks and ethical issues and undertake a [i] hazard identification and risk assessment and [ii] ethical assessment

INTRODUCTION
Clinical significance
The inability to maintain blood glucose within a normal range is a major health problem in both the developed and developing world.  Overt Diabetes mellitus (DM) affects as many as 133 million people, but impaired glucose tolerance affects 2-3 times this number.  DM is classified into:-
•    Type 1 DM – is due to absolute deficiency of insulin and usually presents in childhood. It tends to present as an acute diabetic ketoacidosis and is diagnosed using clinical symptoms and biochemical findings at presentation (ie hyperglycaemia, ketosis, acidosis, coma).

•    Type 2 DM – is the result of a relative deficiency of insulin and usually presents in adulthood.  The progress of this disease can take many years as it moves from normal, through impaired glucose tolerance to overt DM before causing the patient to seek medical help.  The earlier the diagnosis of this condition the better is the long term prognosis for the patient.

Patients suspected of DM, due to family history, suggestive clinical symptoms or equivocal blood glucose results, should undergo an oral glucose tolerance test (OGTT).

Oral Glucose Tolerance Test (OGTT)

The OGTT test should be performed early morning following an overnight fast (8-14hr).  The diet on the 3 days proceeding the test should be unrestricted ie >150 g carbohydrate.  Smoking, eating and physical exercise are not permitted during the test.  The test should not be performed if there are any contra-indications (known drugs that interfere, predisposing illness etc).

On the day of the test a 75g glucose load is administered and blood glucose levels followed over a 2.5 hour period by taking blood samples every 30 minutes.

Blood glucose measurement

Blood, plasma or serum glucose should be measured in a registered laboratory using validated and controlled methods. For the purpose of this practical, hand held glucose meters will be used to demonstrate the technology available for self-monitoring. This is not the recommended method for the diagnosis of DM.

The ‘stick’ technology uses impregnated enzymes to generate an electrical response which is measured by the meter.  The first enzyme oxidises the glucose and in the process forms hydrogen peroxide.  Each molecule of glucose produces one molecule of hydrogen peroxide.  The second enzyme, peroxidase, releases atomic oxygen from the hydrogen peroxide, and this charged atom is detected by the electrode assembly in the strip and a signal sent to the meter.  The electrical signal is proportional to the amount of glucose.

glucose oxidase
Glucose  +  O2                Gluconolactone  +  H2O2

peroxidase
2H2O2                           O-  +  H2O

REAGENTS and MATERIALS

1.    Accu-Chek Advantage blood glucose meter (Roche Diagnostics)

2.    Advatage II glucose test strips (Roche Diagnostics).
Store at room temperature until shown expiry date.

3.    Accu-Chek Advantage II Quality Control (Roche Diagnostics)
Standard QC check solutions: [low] and [high] control glucose solutions.  The acceptable range is shown on the side of the test strip pack.

4.    Disposable safety lancet blood letting devices
Single use lancets [use once only and then dispose into designated sharps bins].

PROCEDURE

First- perform a QC check
1.    Insert the Chek [sic] Strip into the meter so that it turns on automatically and performs a self-check sequence.

2.    If the screen shows OK remove the Chek Strip and the screen shows L1.

3.    Insert a test strip into the meter within 30 seconds and holding the supplied L1 QC check glucose solution bottle horizontally gently squeeze until a drop is formed.  Touch the drop to the side of the strip and allow it to flow into the strip until the meter beeps.

4.    A rotating square symbol on the screen shows that measurement is taking place.

5.    When the reading appears check this is within the acceptable range given on the side of the bottle of strips.

6.    Remove the strip and repeat for the level 2 QC.

Acceptable range
(mmol/l)    Your result
(mmol/l)
Low QC    2.2 – 3.9 (mmol/l)    2.8 (mmol/l)
High QC    15.3 – 20.6 (mmol/l)    17.1 (mmol/l)
Record your QC results:

Blood Sampling Preparation

7.    Wash hands thoroughly with warm water and soap.

8.    Using the safety- lancet

i: Twist off protective cap

ii: Press the safety lancet against the chosen and cleansed puncture site

iii: Press the trigger button

iv: Dispose of the safety lancet immediately in the designated sharps container

v: Gently massage the finger in the direction of the puncture site to obtain the required blood volume

Collecting blood and taking a reading

9.    Remove a single test strip from the container and fit into the slot at the bottom of the meter, so that the blue area is facing up and the meter switches itself on.

10.    Wipe the desired puncture site with a sterile wipe and allow this to dry.  To increase blood flow to the finger and improve the amount of blood obtained, allow your arm to hang down by your side.

11.    Hold the safety lancet device against the skin and press the release button. Immediately dispose of the used lancet in the sharps bin.

12.    Touch the edge of the blue area on the test strip to the blood drop until you hear a beep (this indicates sufficient blood as been collected).  DO NOT PUT THE BLOOD ON THE TOP OF THE BLUE AREA.

13.    Measurement begins and the screen shows a rotating box.

14.    The result is shown in mmol/L after 15-30 seconds.  Record this.

15.    Remove the test strip from the meter and discard into the clinical waste bin.

Performing the oral glucose tolerance test

16.    Follow steps 7-15 to determine the fasting blood glucose.

17.    If the fasting glucose is greater than 10 mmol/l you must not continue with the rest of the test.  If less than 10 mmol/l continue to the next step.

18.    Drink 353 ml Lucozade (this contains the standard glucose load of 75g glucose).  This must be consumed within a 5 minute period.  This is time zero.

19.    Measure the blood glucose at 30, 60, 90 and 120 minutes using steps 8-15 and record the results.  You must sit quietly during the test period.  Smoking and eating are not allowed during the test. Small volumes of water may be consumed, if required.

Interpretation of results.

The full OGGT, when administered by properly trained medical staff, is not only used for the diagnosis of DM, but other disorders of glucose metabolism.  In fact, the World Health Organization (WHO) recommended method only requires the fasting and 2 hour sample to diagnose DM using the following criteria:-
Fasting glucose (mmol/L)
Whole capillary blood        2 hour glucose (mmol/L)
Whole capillary blood
Normal     3.5 – 5.9    AND    3.5 – 6.0
DM    > 6.0    OR    > 11.1
Impaired glucose tolerance    3.5 – 5.9    AND    > 7.8
Impaired fasting glycaemia    >5.6    AND    < 6.1
ASSESSMENT

Note:
For Qs 1 – 3: pay attention to the criteria for good tables and graph-drawing. See any title in the ‘Practical Skills in…….’ series published by Pearson for reminder and/ or your first year skills notes, or equivalent.

For Qs 6 and 8 indicative word count is 500 words.

All Qs [not just Q4!]: you are expected to support your answers with reference to a range of [academic] sources, using any standard parenthetical referencing format. i.e you are expected to undertake a literature search and additional reading in support of this assessment and refer to supporting sources in your answers.

1.    Tabulate all the relevant data for Qs1-3, including your QC results.
Experimental data
QC results:
Low QC- acceptable range 2.2-3.9 mmol/l; result: 2.8 mmol/l
high QC- acceptable range 15.3- 20.6 mmol/l; result: 17.1 mmol/l

OGTT results given in format:  time in minutes (glucose concentration in mmol/l)
0/fasting (5.1); 30 (8.6); 60 (8.9); 90 (7.4); 120 (5.5)

2.    Plot the experimental OGTT results you collected (personal, or observed) on a graph.

3.    Using the information provided in Table 1 also plot on the graph generated for Q 2

[i] the mean glucose concentration and
[ii] the 95% normal reference range [show this as a shaded area].

nb. the 95% normal reference range is calculated as the mean glucose concentration ?2 standard deviations.                                                                                                      (15 marks)
Table 1: Data for normal expected curve.

Time [min]    Mean Glucose Concn. [mMol/L]    Std dev
Fasting     4.7    0.6
30     8.0     1.0
60     8.5     1.2
90     8.2     1.1
120    5.5     0.7

4.    Use a scientific/medical database to find a recent article that compares methods/devices for blood glucose testing. These may be laboratory or point of care devices.

Give the database you used, plus key words and number of hits returned. You should detail how you refined your key words/ search criteria in order to reduce, or increase ‘returns’ to a reasonable number to read through in order to select a good paper. Concisely explain why you chose the specific paper you cite [as opposed to others in the list returned] and summarise the main features of the article in a paragraph. Your summary should be approx. 200 words [you should NOT just reproduce the abstract].                          (6 marks)

5.    Formulate an appropriate hypothesis and null hypothesis for the OGTT.
nb: Remember an hypothesis/ null hypothesis must be specific, accurate and testable [so as to be accepted/ rejected]. i.e. avoid general statements.                                                                    (4 marks)

6.    Define the terms ‘hazard’ and ‘risk’ within the context of scientific practical work.
List the hazards associated with the OGTT in this practical class and explain how the risks associated with each hazard are controlled.                    (10 marks)

7.    List the ethical issues associated with the OGTT in this practical class. Explain how the ethical issues are controlled.                                 (10 marks)

References and citations                                (5 marks)

(total:        /50= ___%)

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