Practical Matters: Differentiate cocci from imposters in the urine
All elements visible in the 10X field (e.g. casts, large cells) and most larger elements in the 40X field (smaller cells, most crystals, most fat droplets) can be differentiated from bacteria by size. However, I, too, often can't distinguish between cocci and other small particles. And using new methylene blue as a wet-mount stain doesn't help me because stain particles mimic cocci bacteria. To confidently differentiate between cocci and other particles, I dry a wet-mount sediment sample and stain it with a modified Wright's stain or Gram stain. Cocci reliably stain this way and are easy to distinguish from other nonstaining particles. Make sure to periodically check your stains, however, as these can be contaminated with bacteria and lead you to an erroneous diagnosis of bacteriuria.
Todd Deppe, DVM, DACVIM (internal medicine)