A burn is an injury to the skin or other tissues caused by heat, electricity, chemicals, or radiation. The severity of a burn is directly related to its size and depth, requiring a rapid, standardized assessment in emergency medicine. Determining the percentage of Total Body Surface Area (TBSA) affected is a foundational step in burn care. This measurement guides important decisions, such as the amount of intravenous fluids needed to prevent shock and whether a patient requires transfer to a specialized burn center.
Understanding the Rule of Nines
The Rule of Nines (RoN) is a method used by emergency medical providers to estimate the Total Body Surface Area (TBSA) affected by second-degree (partial-thickness) or third-degree (full-thickness) burns in adults. This system simplifies the human body for rapid assessment by dividing it into anatomical segments that represent 9% or multiples of 9% of the total surface area.
The RoN assigns distinct percentages to major body regions, totaling 100%. The entire head and neck account for 9%. Each complete upper extremity, including the hand, is assigned 9% of the TBSA. The trunk is split into the anterior torso (18%) and the posterior torso (18%), totaling 36%.
The lower extremities are the largest segment, with each leg, including the foot, representing 18% of the TBSA (36% total). The perineum, which includes the genital area, is assigned the remaining 1%. The sum of these standardized percentages provides a fast estimate of the injured surface area, primarily used for initial fluid resuscitation calculations.
Calculating Total Body Surface Area
The Rule of Nines calculation involves summing the 9% segments corresponding to second-degree or third-degree burns. Only these deeper burns are included in the TBSA calculation because they cause significant fluid loss through the damaged skin barrier. First-degree burns, which are superficial and only affect the top layer of skin, are excluded from this percentage.
For example, a patient with second-degree burns covering their entire left arm (9%) and the entire front of their torso (18%) has an estimated TBSA burned of 27%. This percentage is used to calculate the volume of intravenous fluids required to prevent hypovolemic shock. Since not all burns fit neatly into the predefined 9% sections, a supplementary method is often used for smaller, irregularly shaped burns.
This secondary tool is the Palmar Method, which estimates that the size of the patient’s palm, including the fingers, equals roughly 1% of their TBSA. This technique is useful for assessing burns that are scattered, patchy, or cover an area less than one of the Rule of Nines segments. Using the patient’s own hand as a measuring tool helps emergency personnel gain a more refined TBSA estimate to guide initial treatment.
Defining Critical Burn Injury
The TBSA percentage calculated using the Rule of Nines is a primary factor in determining if a burn requires specialized care at a burn center. For adults aged 10 to 50, a second-degree or third-degree burn involving more than 20% TBSA is considered serious. A burn exceeding 20% TBSA triggers the need for immediate fluid resuscitation to counteract the body’s systemic response to trauma.
The TBSA threshold is lower for vulnerable populations, such as adults over 50 or children under 10. For these age groups, a second-degree or third-degree burn involving more than 10% TBSA is categorized as a serious injury. Additionally, any third-degree burn greater than 5% TBSA in any age group warrants referral to a dedicated burn facility.
Other indicators automatically classify a burn as serious, regardless of its size. These include burns involving specific anatomical areas: the face, hands, feet, genitalia, perineum, or those that cross major joints. Burns to these areas carry a high risk of functional impairment or disfigurement, requiring expert care.
The mechanism of injury is also a determining factor. Any electrical burn, including lightning injury, or any chemical burn is considered serious due to the potential for deep tissue damage that may not be immediately visible. The presence of an associated inhalation injury, which can compromise the airway or lungs, also classifies the burn as a major trauma. Patients with pre-existing medical conditions, such as diabetes or heart disease, are often treated as serious cases because their underlying health issues complicate management and recovery.
Assessing Burns in Infants and Children
The standard Rule of Nines is not accurate for estimating TBSA in infants and young children because their body proportions differ from adults. A young child’s head is proportionally larger, and their legs are proportionally smaller. Applying adult percentages would overestimate the burn size on the head and underestimate it on the legs, leading to incorrect fluid calculations.
To address this anatomical difference, medical professionals use the Lund-Browder chart, an accurate, age-adjusted alternative. This chart adjusts the percentage contribution of the head and legs based on the child’s age in years. Any significant burn in a child should be treated with caution.
Children have a higher surface area to mass ratio than adults, making them more susceptible to rapid heat loss and fluid shifts, which can quickly lead to hypothermia and shock. Due to this vulnerability, the American Burn Association recommends that all second-degree or third-degree burns greater than 10% TBSA in patients under 10 years old be treated at a specialized burn center. This lower threshold reflects the greater physiological risk a burn injury poses to a developing body.