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SUMMER 2008 |
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C L I N I C A L P R A C T I C
E
What does
fasting mean?
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Stock Photo |
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What does fasting mean?
The question may seem a little naïve but it is a very good one when it
comes to some very basic laboratory tests. There are many misconceptions
and some misinformation about fasting that needs clarification.
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By Drs. Edward
Randell & Christopher Kovacs |
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What does fasting mean? The question
may seem a little naïve but it is a very good one when it comes to
some very basic laboratory tests. There are many misconceptions and
some misinformation about fasting that needs clarification. First of
all, fasting implies abstaining from all caloric and nutritive
in-take for a prescribed period of time. In simple terms, absolutely
no food or drink except for water during the entire fasting period.
Avoiding water is not a part of the fasting protocols for most
routine blood work including lipid profiles or diabetes screening.
The recommended length of the fast differs
with the test(s) of interest. For fasting glucose testing, an overnight
fast of about eight to 12 hours is sufficient. A blood glucose measured
early the next morning will provide an adequate sample to screen for
diabetes. But did you know that if the patient fasts beyond 12 hours and
into the afternoon before blood collection that the likelihood of a
false negative screen using fasting plasma glucose will increase? In
fact, as many as 50 per cent of patients with diabetes will have a false
negative result if fasting is continued to the afternoon, compared with
blood collection during the morning. Conversely, the likelihood of a
false positive test by the oral glucose tolerance test for impaired
tolerance or diabetes is increased if fasting is prolonged. During
prolonged fasts the ability to absorb glucose is unregulated.
Things are a little different for lipid
profiles. The two lipid components most affected by the fasting status
is triglycerides and LDL. Because triglycerides are relatively slow to
be digested, absorbed and removed from circulation compared to glucose,
fasting for lipid profiles should be for at least 12 hours. Again, water
is the only fluid allowed during the fast. Interestingly, total
cholesterol and HDL change very little whether the patient is fasted or
not. If in doubt about what an appropriate fasting procedure would be
for a lab test, a good one to remember is: “Absolutely no food or drink
other than water from 8 p.m. onward, and present early the next morning
for blood collection.”
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