A mass casualty kit, also called an MCI kit or MASCAL kit, is a medical kit stocked to treat several injured people at once rather than a single patient. Where an individual first aid kit carries one tourniquet and one set of dressings, a mass casualty kit carries those same lifesaving supplies in multiples, along with triage and casualty movement tools, so a small number of responders can manage many patients until more help arrives.

A mass casualty incident is any event that produces more patients than the available responders and resources can manage using their normal procedures. The number that qualifies depends on the setting: a handful of seriously injured people can overwhelm a single patrol officer or a rural EMS crew. Active violence events, multi-vehicle crashes, building collapses, and industrial accidents are common examples.

The core of any mass casualty kit is bleeding control in quantity: multiple tourniquets, hemostatic or plain packing gauze, pressure dressings, and chest seals, plus gloves for every responder. Most kits add trauma shears, emergency blankets to prevent hypothermia, and triage markers or casualty cards to track who has been treated. Larger kits often include litters for moving patients, and advanced kits may carry airway and chest decompression equipment intended for trained medical personnel.

An IFAK is sized to treat one person and is carried on the body, and most trauma kits are built around treating a single patient at a time. A mass casualty kit is built for several patients at once, typically in a backpack or large bag, and is often organized into separate per-patient modules or throw kits that can be handed off to other responders. That organization lets treatment start on multiple casualties at the same time instead of one after another.

An active shooter response kit is a mass casualty kit built around the injuries most common in a shooting: severe bleeding and penetrating chest wounds. These kits are designed for use in the warm zone, the area where the immediate threat has been contained but not fully cleared, where Rescue Task Forces of medical personnel escorted by law enforcement begin treating and evacuating the wounded. Many include throw kits and lightweight litters so casualties can be treated and moved quickly to a casualty collection point.

Triage sorts patients by the urgency of their injuries so limited responders treat the most critical survivable injuries first. The START system (Simple Triage and Rapid Treatment) assigns each patient a color-coded category: red for immediate, yellow for delayed, green for minor, and black for deceased or expectant, and the SALT system (Sort, Assess, Lifesaving interventions, Treatment/Transport) uses similar categories. Triage tags, cards, or marker panels record each patient's category so the next responder doesn't have to reassess from scratch.

Basic bleeding control supplies like tourniquets, pressure dressings, and packing gauze can be used by anyone with bleeding control training, such as the American College of Surgeons' Stop the Bleed course. Components like nasopharyngeal airways, needle decompression kits, and supraglottic airways should only be used by trained medical personnel. For organizations, a mass casualty response depends on rehearsed roles, so kit purchases should be paired with regular training such as Tactical Emergency Casualty Care (TECC).