Computed lab values on reachable grids¶

PT %, INR and fibrinogen are computed from a finer measurement, so some integers are unreachable and a heaping screen misreads the comb as heaping.

Background¶

• PT % and INR are calculated from PT seconds, measured to 0.1 s.
• Fibrinogen is computed from a clotting time in the same way.

Data¶

• Central-lab PT % (21,143 results screened) and fibrinogen (18,921), from lab_data.csv; pre-operative PT % from clinical_data.csv.

Method¶

• Whole-number heaping screen at multiples of 10 and 5, with expected counts from a local quadratic fit to neighbouring values.
• Inspection of which integers occur at all, and how often, against the local average.

Results¶

• PT % shows a ×10 ratio of 1.09 (z 4.0); fibrinogen a ×5 ratio of 1.27 (z 11.3).
• Each 0.1 s step moves PT % by roughly 1–2 points, so some integers can never occur (98 and 102 almost never appear) and their neighbours carry the extra counts.
• For fibrinogen, 44 integers between 150 and 450 mg/dL occur at less than 30% of the local average.
• Pre-operative PT % shows the same artefact.

Discussion¶

• This is not heaping, and fitting a rounding model would mistake it for one.
• PT % and INR should take their interval from PT seconds (±0.05 s mapped through the local PT–PT % relationship), and use the recorded PT seconds where present.
• Fibrinogen has no underlying time in the data, so its interval should be the local spacing of reachable values, estimated from the data, rather than ±0.5 mg/dL.
• The checklist now checks for unreachable values before reading excesses as heaping, and screens use a local smooth expectation, not neighbour means.

Limitations¶

• The fibrinogen interval has to be estimated from the data, because the clotting time is not recorded.

Tags: from the resolution work