Why does the lab do a combined test?

[jt3002]

I'm asking these questions on behalf of another person. I have read over some of the comments here but I'd like to understand the meaning of the combined test result. The results are from Arup labs: HSV 1 Ab, IgG (Elisa) 0.07, HSV 2 Ab, IgG (Elisa) 0.20 which are both clearly negative. However the combined tests are as follows HSV 1/2 Combined Ab, IgM 0.83 and HSV 1/2 Combined Ab, IgG 1.62. The combined IgG test is labeled as positive. From the discussion here, the IgM test is negative and inaccurate anyway and perhaps the Combined IgG test is inaccurate as well. Is that true? If so, why is this test even performed? The doctor's office recommended a retest in a month and was perplexed as to why the combined test was positive when the individual tests were negative. My understanding is that the exposure was about 16 weeks ago and there have been no blister symptoms. Thanks very much for your insight.  

[Edward W Hook, MD]

The IgG ratio is a mathematical ratio between your IgGs to HSV-1 and 2, calculated using a formula provided by the manufacturer.  As it is of little help to me as a professional I have not spent time worrying about it.  Cosider however that even with 2 negative tests, if the difference between the 2 negative values is large enough, you can still have a high ratio.

If you want more information you will need to go to the lab.  Sorry.  EWH

[jt3002]

Thank you very much for your response.

A few quick follow-up questions for clarity since I wasn't able to find any discussion of this topic with searches of this site. When you say test ratio, what two quantities are we talking about? Does the combined IgG test measure anything other than antibodies to HSV-1 and 2? I've seen much discussion here that the IgM test is worthless, but not as much discussion as to why the combined IgG test is worthless other than that it does not distinguish between HSV-1 and HSV-2 --- are there any other reasons to say it's worthless? Finally, are there are references to which I can point a doctor to, other than saying those dreaded words, "I read it on the internet". Thanks very much for your response in advance. This will be my last post on the subject.

[Edward W Hook, MD]

The test manufacturers suggest reporting lab results out in the fashion you suggest.  They do so because they suggest that the test ratio may be helpful.  the fact of the matter is that this is not the case.  As for your "friends' test results, here is the bottom line:

You do not have HSV-1 or HSV-2.  The IgM test is worthless but is negative as well.,  The ratio is meaningless as well and should be ignored.  My life would be far less complicated if they didn't present these sorts of meaningless results. The person whose blood was tested does not have herpes.  Take care.  EWH