Muddy brown casts are diagnostic for ATN, or acute tubular necrosis, which is an intrinsic form of AKI. But if you don’t see muddy brown casts, can ATN be ruled out? The literature on this is surprisingly light. There is one study of 267 patients with AKI who were classified as low or high pretest probability for ATN and had urine samples analyzed. The study concludes: “In patients with a low pretest probability of ATN (initial diagnosis of prerenal AKI), lack of casts or RTEC on urinary sediment examination had a sensitivity of 0.73 and specificity of 0.75 for a final diagnosis of prerenal AKI. The negative predictive value of lack of casts or RTEC in patients with low pretest probability of disease was 91%.” One important point to take away is that your pretest probability matters a lot for how confident you should be about making a diagnosis.
This review, by the same group, has a nice discussion section about making the diagnosis of ATN through a combination of methods, including microscopy, and use of FeNa and FeUrea.