Question:
Do I have to fast before the blood withdrawal – if so, for how long?
Answer:
It is sufficient if the patient has fasted 2-4 hours prior to the blood withdrawal. A 12 hour-fasting which is necessary in laboratory diagnostics to determine quite a number of parameters is not mandatory for IgG tests. Yet is has to be considered that blood samples having been taken shortly after a meal are often lipemic. These samples show a considerably worse test variance. It should also be recommended to the patient that the last meal taken before the blood withdrawal should not have been a fatty meal. In practice, this means: There is no problem if e.g. the patient has had a normal breakfast at 8:00 o’clock and comes for the blood withdrawal at 11:00. Yet it is not advisable if the patient has had a heavy lunch at 13:00 and comes for the test at 15:30.
To summarize the recommendations to the patient:
- Fasting of 2-4 hours before the blood withdrawal
- No fatty meal before the test
Question:
(Argumentation of lab physician) Isn’t it advisable that the blood samples be taken from persons who have at least fasted overnight? – There seems to be an important increase of interferences in the analyses if that is not the case. That strongly reduces the efficiency of our tests and thus the probity of their results’.
Answer:
This is an ongoing discussion and reflects the opinions of marketing people versus lab specialists and German trained physicians. Marketing people tend to make it as easy as possible for the patient to generate more samples, lab specialists want to have the best results without any unnecessary interferences. French trained specialists always ask for fasting samples, German specialists even accept samples for substrates from non-fasting samples, because in France patients go to the lab to get the blood drawn and in Germany they go to their physician during consultation hours. Lab physicians always want to have the best pre-analytical quality and this means a fasting serum sample. In general, this is justified. But in the case of IgG testing there is a consensus that overnight fasting is not necessary. It must be guaranteed that no interfering substances are present in the blood when the sample is drawn. In ELISA testing these are mainly lipids. Therefore a 2-4 hour fasting is sufficient to eliminate all circulating lipids from circulation, if no lipid metabolising disorders are present.
(Answers are by Dr. Camille Lieners, Scientific Advisor ImuPro)