[Question #13713] Fasting and Test Accuracy

1 months ago

Hi Doctor,

Just wanted to check something quickly regarding fasting and blood/urine tests.

I’ve had several tests done, including blood tests such as HIV Ag/Ab, HIV RNA, HCV Ab, HCV RNA, RPR, TPHA, HSV IgG, HBsAg and Anti-HBs.
I’ve also had urine STI PCR testing for Chlamydia, Gonorrhea, Mycoplasma/Ureaplasma, and Trichomonas.

Some of these tests were done without fasting, some after short fasting (around 2 hours), and some after longer fasting( 8 hours). I’ve also seen some websites mention that 8–12 hours of fasting is needed, while others (including my clinic) say fasting is not required. So I'm a bit confused.

I’d like to clarify a few points:

Is fasting actually required for any of these tests, or no fasting at all is okay and does it not affect the accuracy at all?
In your usual practice, do you recommend fasting for these tests?
Is there any real difference in accuracy between fasting and non-fasting samples? Is one better than the other?
My most recent (and final) blood test was done after fasting for about 4 hours. Do I need to repeat the test with longer fasting, or can I rely on the result?

Thanks very much for your advice.

H. Hunter Handsfield, MD

1 months ago

Welcome to the forum. I'm happy to help.

No tests for STI or HIV -- including all the tests you name -- is affected by food intake or fasting. In other words, fasting or not makes no difference at all.

Your four questions all basically are the same, just different wordings. To be explicit:

1. Fasting makes no difference at all.

2. No, I never advise fasting for these tests. It isn't necessary.

3. No, there is no difference in test accuracy or timing on the basis of food intake.

4. No, those tests do not need to be repeated. You can rely on the results.

So no worries about your test results. Let me know if anything isn't clear.

HHH, MD

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H. Hunter Handsfield, MD

1 months ago

By the way, fasting is not required for the large majority of lab tests. It often is necessary for blood glucose, lipids (cholesterol etc), and a few other tests -- but not for most tests and never for any tests for any infection.---
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