Collateral history is the information you obtain from people other than the patient — family, friends, prior providers, records. Often it's the piece that closes a diagnostic question or reveals what the patient couldn't or wouldn't tell you. Used well, collateral substantially improves accuracy and treatment planning.
When collateral matters most: limited insight (the manic patient who insists everything is fine, the early dementia patient who doesn't perceive their cognitive decline), capacity questions, cognitive concerns, long-standing patterns the patient sees as baseline, severity assessment that the patient may underestimate or overestimate, and disagreement between presentation and referral source.
Patient consent first in routine outpatient situations. Document the release. In acute situations — capacity assessment, danger to self or others, severe cognitive impairment — collateral may proceed without explicit consent under emergency exception, but document the rationale.
Specific questions beat open-ended. "Tell me about your father" produces narrative; "What's different about your father compared to a year ago?" produces clinically useful change data. "Have you noticed any safety concerns at home?" produces direct answers; "How is your father doing?" produces polite responses. Ask the questions that will actually inform the assessment.
Discrepancies are diagnostic data. The patient who reports being "fine" while family describes substantial functional decline isn't necessarily lying — they may have limited insight, may be minimizing for cultural reasons, may not perceive what others see. The gap between accounts is information, not something to resolve by picking one side.
Document the source and the consent. "Collateral from spouse (verbal consent obtained) reports: increased forgetfulness over 6 months, withdrew from social activities 3 months ago, recent driving incident causing family concern, denies major financial errors." Note source, date, what was asked, what was answered. The structure protects the patient and the chart.
Sometimes the collateral source itself is the clinical issue. The over-involved family member who answers every question for the patient. The family system organized around the patient's illness. The history of conflict or abuse that explains current family dynamics. Notice these patterns; they shape what you can learn and what treatment will need to address.