What this case supports, and what it does not

The guided and practice cases support one thing clearly: with a correctly paired, appropriately timed serum and fluid sample, the protein ratio, LDH ratio, and absolute LDH comparison can be calculated and checked against Light's published thresholds. Both worked examples land on defensible calculations because the containers, timing, and pairing were handled correctly upstream of the calculation.

None of these numbers is portable. Light's criteria classify pleural fluid only; a peritoneal specimen needs its own framework such as the serum-ascites albumin gradient, and reusing pleural cutoffs there would misclassify the specimen. A chemistry result on a fluid matrix that has not been verified is not equivalent to the same test on blood, even when the number looks reasonable; it is not reported from this method for this fluid until that verification is complete. A separate, narrower gap is a missing fluid-specific reference interval on an already-verified method, which can be released with a comment that the interval is unavailable so the number is read against trend or the clinical picture rather than a numeric range. Conflating the two lets an unverified result slip out as a routine value with a minor caveat, which is exactly the release the verification requirement is meant to prevent.

Several fields are explicitly local rather than universal: the exact analyzer, reagent family, and body-fluid application for cell counts and chemistry; the reportable range, dilution rules, and manual-review triggers; the reference intervals and their units; and the critical-value, escalation, and turnaround-time policy for a pH or count that needs urgent communication. None of those were specified here because they depend on the performing laboratory's own controlled procedures and validation files, and stating a specific number for any of them without checking that documentation would be unsupported.

A pleural fluid classification is only as strong as its label, its pairing, and its matrix validation; when any of those three is missing, report the number with its limitation instead of the conclusion it would otherwise support.