Four tubes, one clock

A lumbar CSF collection arrives at the bench at 10:12, seventeen minutes after the documented 09:55 collection time. Four tubes are labeled 1 through 4. Available volume is 0.4, 0.6, 0.5, and 0.5 mL, well short of a generous specimen. Tube 1 is visibly blood-tinged. The requisition asks for cell count, protein and glucose, and a culture.

The laboratory cannot draw more fluid. Every microliter has to go where it does the most good, and the cell count has to be read before cells start to lyse or settle out. Whoever picks up this specimen has to decide, in the next few minutes, which tube goes where and how fast the cell count needs to move.

The decision is worked using one case: allocate the tubes, set up and read a Fuchs-Rosenthal chamber, calculate two results, and recognize when a count is not trustworthy enough to release.

Treat every CSF specimen as low volume and time-sensitive until proven otherwise, because by the time cells have visibly settled the count is already compromised.