The recently commercialized PTW microDiamond detector (T60019) has been designed for use in small radiation fields. Here we report on the measurement of relative output ratios for small fields using five microDiamond detectors. All of the microDiamond detectors over-responded in fields smaller than 20 mm, by up to 9.3% for a 4 mm field. The over-response was independent of accelerator type and choice of collimation. The over-response was slightly larger than that observed in silicon diodes. Since all five microDiamond detectors showed the same over-response the corrections presented here should be transferable to other examples of the microDiamond detector, provided that the detector meets the manufacturing specifications and the beam characteristics are comparable.