I recently received a letter from a gentleman who had done both the Southwest Naturopathic College serotype blood panel (which uses Lewis blood grouping for the determination of secretor status) and the North American Pharmacal secretor test (which uses saliva). He writes:

"....this last April I did the saliva secretor test from North American Pharmacal and tested as a secretor. I have since done the serotype panel from the Southwest Naturopathic College and was typed as a Lewis (a-b-) 'Non-secretor.' How could this be? Did the NAP test err?"

In order to understand the apparent discrepancy, we must understand the basic differences between both methods and the results they yield.

The NAP saliva test looks for the presence of ABH antigens in saliva. This is accomplished by a process called 'antibody inhibition': the saliva is first incubated with antibody; then fresh red blood cells of that person's same blood group are added; if the person is a secretor the antibody would have been absorbed by their own ABH antigens and the new cells will not clump. If the person is a non-secretor they will not have the antigen present in their saliva to bind the antibody; instead, the added cells will clump. (Figure 1)

Figure 1. The basics of salivary inhibition testing. Two saliva samples (light blue) of unknown secretor status (left and right) are tested by first adding the appropriate antibody for that person's ABO group (1). If the saliva is that of a secretor (left column) the saliva will contain ABH antigens (brown) and the antibody (purple) will bind with them. If the person is a non-secretor, the antibody will have nothing to bind with, and will remain in a free state (2). Next, red blood cells of the person's ABO group are added and a reaction, if any, is noted. If the sample is from a non-secretor (right column) that antibody (still unused) will now bind to the free antigen on the red blood cells and clump them. If the sample is from a secretor, the antibody will have already been used to bind the person's own free antigen and the cells will remain unclumped (3).

Because saliva testing is actually assessing the presence or absence of ABH antigen in saliva, it is the only method to determine secretor status correctly in all populations.

There is, however, another method using blood and testing for a blood grouping system (the Lewis groups) that can be used to infer secretor status, but suffers from the inability to reliably determine secretor status in 100% of the population.


Lewis blood groups share some genetic links with ABO. In the Lewis system, roughly 90-95% of us (depending on race) start out making an antigen called Lewis (a). Because of a genetic link, those of us who are secretors then convert all of our Lewis (a) into Lewis (b), whereas non-secretors lack the ability to do this. Thus, if we Lewis type an unknown sample of blood and the result is Lewis (a-b+) we can infer then that person is a secretor, Conversely, if the sample types as Lewis (a+b-), we infer that that person is a non-secretor.

Pretty neat, except for one complication.

Anywhere from 2-5% of the population are so-called 'Lewis double negatives' meaning that they do not manufacture Lewis (a) and so cannot make Lewis (b). In this category of individuals (including yourself) Lewis typing cannot be used to infer secretor status; only saliva will do. (table 1)

Lewis Group Secretor Status
Lewis (a+b-) Non-secretor
Lewis (a-b+) Secretor
Lewis (a-b-) Cannot determine
Table 1. Lewis subtypes and secretor inference.

So why did the serotype panel tell you that you were a non-secretor?

Frankly, because I have said to do it.

The Lewis typing for secretor determination is a 'quick and dirty' method of secretor determination; that is the nature of the beast. So in those individuals who are double Lewis negative (a-b-) I have advised that it is best to consider them 'non-secretors,' (since they do share many of the same disease susceptibilities and metabolic disturbances of non-secretors) until saliva testing discloses otherwise.

In this gentleman's case, the saliva testing, versus the Lewis typing, yielded the more accurate result of secretor status, but knowing he is a double Lewis negative is a valuable piece of information as well.