21 hours ago, diplomatic_scarf said:

IgG is only a monomer antibody and won't cause agglutination visible to the naked eye. 

Sorry, you must be fed up with me.  I know I am like a dog with a bone, but, although IgG is a monomer antibody, it does have a valency of two, which is why, occasionally, it can cause agglutination visible to the naked eye.  Notably, some examples of IgG anti-D cause agglutination at 37oC with D--/D-- red cells, with no potentiator.

Yes, you are completely right Malcolm and others.  My experience is limited to common positive DAT results for OB patients, which is the ABO mismatch. Those positives are usually 1+ or weaker. That's why we always do a microscopic if we get a macroscopic negative.  I forgot that Rh mismatch DATs can go from 2+ to 4+ positive, because I hardly ever come across those.  Thank you. 

Read your circular of information for your specific antiglobulin reagents. Bio-Rad reagents are very specific. If no agglutination is observed macroscopically, it says, "Negative reactions may be examined with an agglutination viewer, however, microscopic examination is not recommended."

 

The original question was for all DATs, and this is from a reference lab perspective. We us Quotient for all three reagents. We use CAP. No we only VERY rarely use the scope, and it's at 10x rolling a tube and it's just for informational purposes. We also perform our DATs in Gel, with a significant difference between reaction strengths. Many times the tube is negative and the Gel is 1+ or 2+, so weak reactions should be caught there. Many identified Abs in eluates from tube negative, Gel 1+ samples. We HERE define optical aid to be a lighted mirror for tubes or laying the gel cards on an old school view box. (that doesn't mean it's the right way, just our way)  We have made an addition to our SOP to never use a mirror to read Gel cards. We allowed one tech to do this for almost a year and the amount of extra work was staggering with not a single clinically significant Ab identified. (that doesn't mean it's the right way, just our way) We don't use the scope because after many years of experience, (here) we determined that we were not viewing anything of clinical significance. I think that is the key, you have to balance risk/reward. If you use the "we have to do what's safest" theory, then you might end up running full panels and FICIN panels on negative screens.