Lots of ideas presented here. Basically there are these possibilities and they all need to be considered/investigated: 1. Specimen from prenatal/previous testing was mislabeled, e.g. someone else's blood. 2. Tech made an error somewhere along the testing path or recording during the previous testing. 3. Tech made an error somewhere along the testing path or recording during this testing. 4. Sample 'today' is the 'wrong' patient ... misdraw, misidentification or maybe another patient who is not who she says she is (e.g. using someone else's insurance card, it happens a lot in my area). 5. You are picking up a large fetal-maternal hemorrhage. 6. Patient is Rh Pos (weak D). Possibility 1 + 2: Cannot be determined at this time. Possibility 3 + 4: I'm assuming you've investigated these already. Possibility 5: A KB Stain and/or Fetal Hemoglobin Assay could answer this question. Possibility 6: The content of Anti-D reagents differ quite a bit, so we cannot expect the RBCs from some patients to elicit the same results if we change reagents. The 'Ortho vs Immucor' reagent difference has been expressed in here, but this goes for other manufacturers as well. Check out the information in the Quotient website about their various Anti-D reagents (beta, optimum, delta, etc.) as an illustration of the vast differences of detection capabilities. And this includes, as someone brought up in here, technique and timing. It all depends on what you want to see (DVI, for example) and how you want to see it. (Note that some Anti-D reagents do not require an AHG phase to detect 'weak D', some don't even require an incubation, some pick up DVI while others won't, etc.) So, in order for you to address this possibility, you need to find out what reagents were used and what is their capibility. 'nuff said? Did I forget anything?