Index Case¶
A recognized case episode or affected person designated as the entry point for an inquiry into related cases, without thereby identifying the earliest case or causal source.
Core Idea¶
An index case is a recognized case episode or affected person designated as the entry to an inquiry into related cases. The designation tells investigators where to start; it does not identify the first affected person, the source of a condition, or a transmission direction. A case episode and the patient in whom it occurs may be counted differently in partner services.[^ref-50bde03ee1ee]
Scope of Application¶
- Infectious-contact inquiry: In a New York City HIV report, a newly diagnosed and reported patient was called the index patient. Investigators contacted named partners and compared genotype records. They could not establish a definite transmission chain or how he acquired infection.[^ref-6c1e5694bce3]
- Familial cascade inquiry: In the CATCH trial, genetically confirmed familial-hypercholesterolemia index patients were referents for offering testing to eligible first-degree relatives. The trial had 87 index patients, 359 eligible relatives, and 99 tested. A positive relative could become a new case patient in a later permitted cycle; infection is not the relation here.[^ref-994654635a6a]
Both settings require a defined inquiry, a recognized clinical case or patient, its designation as entry, and a question about related people. Neither requires that the entry be causally or temporally first.
Clarity¶
The first recognized or reported outbreak case need not be its source. “Index” can also mean the first case published in medical literature, but publication priority is a different claim. A generic graph seed lacks the clinical case, ascertainment, and related-case purpose.[ref-50bde03ee1ee][ref-6c1e5694bce3]
Manages Complexity¶
Keep the starting case separate from the people later found, and keep recognition order separate from origin. This prevents the NYC contact and genotype matches from being read as a proved chain, or the CATCH family referent from being treated as the first affected relative.[ref-6c1e5694bce3][ref-994654635a6a]
Abstract Reasoning¶
The portable pattern is a recognized entry object relative to a defined inquiry, followed by extension toward related objects while origin remains open. The clinical case or patient and medical ascertainment make this named role domain-specific. Designating the entry can precede any completed network traversal or selection among alternatives.
Knowledge Transfer¶
When reading an investigation, ask what made the case recognizable, which related people the inquiry seeks, and what evidence would be needed to infer origin. Apply the same questions to contact investigations and family cascades, while retaining each setting's distinct relation and evidence rules.[ref-6c1e5694bce3][ref-994654635a6a]
Neighborhood in Abstraction Space¶
Index Case sits in a sparse region of the domain-specific corpus (84th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.
Family — Unclustered & Miscellaneous (2721 abstractions)
Nearest neighbors
- Case-Definition Drift — 0.82
- Clinical Endpoint — 0.82
- Watchful Waiting — 0.81
- Event Sampling Methodology — 0.81
- Face — 0.81
Computed from structural-signature embeddings · 2026-10-17
References¶
[^ref-6c1e5694bce3]: CDC and collaborating investigators, Investigation of a New Diagnosis of Multidrug-Resistant, Dual-Tropic HIV-1 Infection—New York City, 2005, Morbidity and Mortality Weekly Report 55 (2006). Original full report; opening and Case Report, Contact Investigation, Laboratory Reporting, Sequencing, and Editorial Note. The investigation names an index patient and leaves acquisition and a definite transmission chain unresolved. [^ref-994654635a6a]: Nanchen et al., Genetic Cascade Screening for Familial Hypercholesterolemia: A Randomized Clinical Trial, JAMA Network Open (2026), DOI 10.1001/jamanetworkopen.2026.6100. Original full text; Abstract, Participants, Cascade Procedures and Family Trees, Control and Intervention, and Results. The 87/359/99 counts and up-to-three-cycle rule are trial-specific. [^ref-50bde03ee1ee]: CDC, Recommendations for Partner Services Programs for HIV Infection, Syphilis, Gonorrhea, and Chlamydial Infection, Appendix B, “Index case” and “Index patient,” Morbidity and Mortality Weekly Report 57(RR-9) (2008). First-party operational glossary for HIV/STD partner services; its episode-counting convention is scope-specific.