On the top of my head I can't remember the particular article but I know that studies in diffuse brain injuries are looking at ramified microglia as potential markers.  Sorry I don't have specifics but perhaps look at some of the work done by Jonathan Lifshitz.  Hope this helps.

Iba1 will stain both ramified and ameboid microglia, although expression levels will increase with activation. ED1 (for rat) or CD68 will only identify microglia that are activated. 

I have used Iba1 and CD11b. They stain both the resting and activated microglia. At the end of the day, it requires careful histological interpretation to differentiate the types of microglia.

I have used anti-CD11b, it works quite well. You should give it a try. Cheers.

Dear David,

All antibodies against microglial cells can  provide the images where  you can figure out activated or in inactivated state of these cells.  Marker Iba1 is the best, which is upregulated during activation. Good luck. Tatyana

Thank you all for answering my question, much appreciated!

I am certain that there are plenty of published articles on the subject, perhaps there is a consensus. I am certain you have looked though. My experience with active versus resting ("inactive" is a misnomer, I wouldn't use it) has been with IBA1. IBA1 expression increases in "active cells". The expression level of IBA1 is relative to "inactive" cells. So unfortunately it doesn't serve as a very good marker.

The answer also depends on the experimental system you are using. In vivo versus primary cell culture. If you are using an in vivo model then probably best to classify your cells by morphology instead of strictly sticking to markers. With primary cells, you're probably better off using markers but cell morphology may be useful. There are several good papers describing microglial morphology and activation state. Best of luck!

As far as I know all markers of microglial cells such as Iba-1, CD11b, CD68, OX-42 are used for the labelling both resting and the activated microglial cells. I have never met the marker of unactivated microglia, I figure out the stages of microglia by morphology.