In progress
Summary and worked examples to follow.
The Four-Square Prioritized Differential (FSPD) is a method for prioritizing a differential diagnosis along two axes — likelihood and degree of harm if not recognized — so that “must not miss” diagnoses are handled explicitly and early, and not buried in a long list. The point of the FSPD is to make explicit what is probable rather than what is possible.
The degree of overlap between a working diagnosis (in the form of a problem representation) and the classic presentation of a disease: this defines probability of the patient at hand having this diagnosis. Draw a 2 x 2 square and label the boxes as Type I, Ib, II, and III:
| Likely | Unlikely | |
|---|---|---|
| Highest probability | I | Ib |
| Lower probability | II | III |
Never forget to consider 1b diagnoses: imminent threats to life if not recognized or acted upon within minutes to hours.
Related: Diagnostic schemas.