A field with two long, pale structures

A morning clean-catch urine specimen is examined at the bench. Dipstick shows specific gravity 1.025, pH 5.5, protein 1+, blood negative, and leukocyte esterase negative. Under the microscope at 100x brightfield, two long, pale, roughly parallel structures sit near each other in the same field. One has straight, parallel sides and rounded, blunt ends and barely shows up until the illumination is turned down. The other is wavy, changes width along its length, and never resolves into a bounded shape no matter how the focus or light is adjusted.

Only one of these two structures is a cast. The question on the bench is not whether something long and pale is present. It is whether that structure has a continuous matrix that was molded by a tubule, because that molding is what separates a true cast, reportable as a distinct sediment finding, from a look-alike.

The work from that field forward: what a cast actually is, why it looks the way it does, how to tell it from the things that mimic it, and how to sort the categories a laboratory reports once a structure has passed the test of being a true cast.

Before naming a category, confirm the structure is a cast at all, because category names mean nothing if the base call is wrong.