Day 17 🧪 Welcome back to 30 Days of ASCP: Make It Make Sense because once you get it, you never forget it. MSU and CPPD crystals both show up in joint fluid during inflammatory arthritis, but the key to the exam is knowing how to tell them apart under polarized light. MSU crystals, seen in gout, are needle-shaped and show negative birefringence: yellow when parallel, blue when perpendicular. CPPD crystals, seen in pseudogout, are rhomboid and show positive birefringence: blue when parallel, yel