There are both secretory IgA and serum-based IgA antibodies. The latter is a blood-based antibody that is measured in the serum. It is measured the same way as the IgM, IgG, and IgE antibodies. IgM antibodies are made in the immune system. IgM is found predominantly in lymph and works to counter viral and bacterial infections. IgG is found in fluids and tissues and is hugely abundant in the body. IgG is one of the first lines of defense against bacterial and viral infections.

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IgA works on the mucosal surfaces. Like the secretory IgA, IgA is particularly crucial for the ears, nose, throat, vagina, bladder, urinary tract, and any other mucosal surfaces.

In my opinion, if you want to determine digestive IgA levels, a stool test is required. You can do a saliva test to determine secretory IgA in the mouth. A blood test for serum IgA is also useful. It can be helpful to do both a fecal IgA and a blood IgA and compare the results. Sometimes you will see a pattern where both the fecal and blood IgA antibodies are low, or both are high.

Sources:

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1782559/
• https://www.immunology.org/public-information/bitesized-immunology/receptors-and-molecules/immunoglobulin-iga
• https://onlinelibrary.wiley.com/doi/pdf/10.1002/path.1877
• https://www.ncbi.nlm.nih.gov/pubmed/30558634
• https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0106750