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Value Added
Variations in Glucose Values in the Laboratory
The estimation of blood glucose values is most important
in the detection and monitoring of diabetes.
 The
world's largest diabetes epidemic is threatening India, which is ill equipped
to cope. Diabetes is the single-most important metabolic disease, which can
affect nearly every organ and system in the body. It has been projected that
300 million individuals would be affected with diabetes by the year 2025. In
India, it is estimated that this deadly disease affects presently more than
30 million people, which is likely to go up to 57.2 million by the year 2025.
The estimation of blood glucose values is most important in the detection and
monitoring of the disease. We find that there are many variations in the glucose
results.
The following are the conditions when you find that the glucose values show
lot of variations:
Clinical Significance: Intra individual fluctuations
in blood glucose concentration are bigger than those of other blood parameters
due to dependence on muscular activity and the time interval since food intake.
The fluctuations are further increased in the presence of deregulation such
as insulin deficiency or hyperinsulinism. For interpreting blood glucose values
in the range of 40-200 mg/dL, attention must therefore may be paid to the following:
- Whether the blood sample was obtained under standardised
conditions, i.e. after a 12-hour period of fasting, pre-pandially, or two
hours post-prandially, or whether the specimen is a non-standardised, random,
non-fasting blood sample.
- Which sample was used for the determination of glucose;
capillary whole blood, venous whole blood, plasma, or serum.
- Which is the level of diagnostic significance for
each individual value. Since the day-to-day variation of blood glucose determinations
may amount to five per cent, a deviation only >11 mg/dL at the upper reference
limit is significant for the diagnosis of diabetes mellitus, for example,
when the venous plasma level is 115 mg/dL.
Medications: Not only anti-diabetic drugs and its
timings affect the blood glucose values, but also some medications such as M-DOPA,
Vit C, Novamine Sulfone can cause decrease of the glucose concentration by up
to 50 per cent, while steroids can cause increase of the glucose concentration.
Specimen: Serum or plasma, free of heamolysis, is
the specimen of choice. Since glucose in whole blood at room temperature can
undergo glycolysis at a rate of approximately five per cent per hour, the sample
should be centrifuged and removed from clot or cells as soon as possible. Glucose
in serum or plasma separated from blood cells is stable for up to three days
at 2 to 8 C. Fluoride has been used as inhibitors of glycolysis to preserve
blood that cannot be separated rapidly.
There are two major groups of methods to estimate
glucose in blood:
- Chemical methods: Most older established
methods for measurement of blood glucose were based on the ability of
glucose to directly reduce chemicals. These methods lack specificity
and cannot be automated. These methods are of historic importance only.
- Enzymatic methods: The most commonly
used for glucose analysis employ enzymes as reagents to increase analytical
specificity. These procedures can be automated with resulting high specificity
and precision.
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Calibration and Quality Control
It is of utmost importance to calibrate and control the glucose estimation in
the laboratory to avoid variations.
Calibration: It is the measure source of variation
in the estimation of the blood glucose. Especially the calibrators used are
very critical. Ideally the calibrators should have the same matrix as the specimens
being tested. The values assigned to the different parameters of calibrators
must be traceable so that variations across the laboratories could be minimised.
Or there would be differences in the results obtained from the different laboratories.
Also it is important to check that the proper calibration is performed and the
calibration is suitably verified.
Quality Control: Once the testing system is calibrated,
it is of utmost importance to see that the system is stable and not fluctuating.
Employing porper quality control/Quality Assurance programme could monitor the
stability and reproducibility of the testing system. This means that there should
be proper internal quality control programme and external quality assessment
programme. These programmes would control the variability and accuracy of the
result.
Also, laboratory accreditation programme (third party assessment) ensures that
the laboratory has an overall quality system in the lab to perform and monitor
all the activities in the lab, also the competence of the laboratory is assessed.
The writer is an expert from Wellspring Laboratories
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