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Total body surface area

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Total body surface area (TBSA) is an assessment of injury to or disease of the skin, such as burns or psoriasis.

In adults, the Wallace rule of nines can be used to determine the total percentage of area burned for each major section of the body.^[1]

In burn cases that involve partial body areas, or when dermatologists are evaluating the Psoriasis Area and Severity Index (PASI) score, the patient's palm can serve a reference point roughly equivalent to 1% of the body surface area.

For children and infants, the Lund and Browder chart is used to assess the burned body surface area. Different percentages are used because the ratio of the combined surface area of the head and neck to the surface area of the limbs is typically larger in children than that of an adult.^[2]

Typical values for common groups of humans follow.^[3] (Due to rounding, values may not add to 100%.)

**Adult**
Anatomic structure	Surface area
Anterior head	4.5%
Posterior head	4.5%
Anterior torso	18%
Posterior torso	18%
Anterior leg, each	9%
Posterior leg, each	9%
Anterior arm, each	4.5%
Posterior arm, each	4.5%
Genitalia/perineum	1%

**Child**
Anatomic structure	Surface area
Anterior head	9%
Posterior head	9%
Anterior torso	18%
Posterior torso	18%
Anterior leg, each	6.75%
Posterior leg, each	6.75%
Anterior arm, each	4.5%
Posterior arm, each	4.5%
Genitalia/perineum	1%

**Adult, obese >80 kg**
Anatomic structure	Surface area
Head and neck	2%
Anterior torso	25%
Posterior torso	25%
Leg, each	20%
Arm, each	5%
Genitalia/perineum	0%

**Infant <10 kg**
Anatomic structure	Surface area
Head and neck	20%
Anterior torso	16%
Posterior torso	16%
Leg, each	16%
Arm, each	8%
Genitalia/perineum	1%

References

[edit]

^ Burn Percentage in Adults: Rule of Nines
^ Types of Burns Archived 2008-02-02 at the Wayback Machine
^ O'Sullivan, Susan B., Schmitz, Thomas J. Physical Rehabilitation. 5th ed. F.A. Davis Company, Philadelphia, 2007. p. 1098, Fig 27.9.

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Total body surface area (TBSA) is the estimated percentage of an individual's total skin surface affected by injury, such as partial- or full-thickness burns, or by dermatological conditions like psoriasis. This metric is fundamental in clinical practice for evaluating the extent and severity of skin involvement, guiding therapeutic decisions including fluid resuscitation, nutritional support, and surgical interventions. Accurate TBSA assessment directly impacts patient outcomes, as it informs prognosis and resource allocation in burn care.^[1]^[2] Several standardized methods exist for estimating TBSA, each suited to different clinical scenarios and patient demographics. The Rule of Nines, introduced by Pulaski and Tennison in 1949 and refined by Wallace in 1951, divides the adult body into sections representing multiples of 9% of the total surface area: the head and each upper extremity account for 9%, the anterior and posterior trunk for 18% each, each lower extremity for 18%, and the genitalia for 1%. This method facilitates rapid triage but is less precise for children or obese individuals due to proportional variations.^[1] The Lund and Browder chart, developed in 1944, offers greater accuracy by segmenting the body into finer regions and adjusting percentages based on age, making it particularly valuable for pediatric patients where the head constitutes a larger proportion of TBSA in younger children. It remains the gold standard in burn centers for detailed assessments, though it requires more time than simpler tools.^[3] For quick estimations, especially of scattered or minor burns, the palmar method equates the surface area of a patient's palm (including fingers) to approximately 0.8–1% of their TBSA, allowing clinicians to sum affected areas by counting palm equivalents. This approach is advantageous in emergency settings but may introduce subjectivity for larger injuries.^[4] In clinical management, TBSA calculations underpin critical protocols, such as the Parkland formula for fluid resuscitation, which recommends 4 mL of crystalloid solution per kilogram of body weight per percentage of TBSA burned over the first 24 hours (with half administered in the initial 8 hours post-injury). Overestimation or underestimation of TBSA can lead to complications like fluid overload or hypovolemia, emphasizing the need for validated tools and trained assessment.^[1]^[2]

Definition and Importance

Definition

Body surface area (BSA), also known as total body surface area (TBSA), represents the estimated total external surface area of the human body.^[5] This measurement accounts for the skin's coverage over the entire body, excluding internal surfaces or orifices, and provides a quantifiable proxy for the body's interface with the external environment.^[5] BSA is expressed in square meters (m²), with average values ranging from approximately 1.6 m² for adult women to 1.9 m² for adult men, depending on individual anthropometric variations.^[6] It is derived from basic anthropometric data, such as height and weight, which allow for indirect estimation rather than direct measurement, as physically tracing the body's contours would be impractical.^[5]^[7] The primary purpose of BSA is to serve as a standardized metric for scaling physiological processes across individuals of varying sizes, facilitating comparisons in areas like metabolic rate, heat dissipation, drug metabolism, and the assessment of injury extent.^[5]^[6] For instance, it normalizes calculations for cardiac output to derive the cardiac index or adjusts chemotherapy dosages to body size, ensuring more precise medical interventions.^[5] This foundational role underscores BSA's utility in clinical practice, where accurate scaling is essential for patient safety and treatment efficacy.^[6]

Clinical Significance

Body surface area (BSA) plays a pivotal role in various medical assessments, including normalizing physiological parameters and scaling treatments. In burn care, the percentage of total body surface area affected (%TBSA) quantifies burn severity, influencing prognosis, treatment planning, and resource allocation.^[8] Physiologically, BSA serves as a superior proxy for metabolic mass compared to body weight alone, as it correlates more closely with basal metabolic rate (BMR) and total body water, being less distorted by adipose tissue variations that disproportionately affect weight-based metrics. This relationship stems from early observations that heat production and dissipation are proportional to surface area, enabling BSA to better reflect underlying physiological processes like organ blood flow and extracellular fluid volume. Consequently, BSA provides a standardized framework for inter-individual comparisons, reducing variability in assessments of metabolic demands and circulatory efficiency across diverse body sizes.^[5]^[9] In clinical practice, BSA is routinely used to normalize metabolic rates and cardiac output for equitable evaluations; for instance, dividing cardiac output by BSA yields the cardiac index (typically 2.5-4.0 L/min/m²), which assesses heart function independently of body size and aids in diagnosing conditions like heart failure. This normalization is particularly valuable in pediatrics and obesity, where weight alone skews interpretations, allowing clinicians to compare physiological performance accurately. Similarly, in oncology, BSA underpins chemotherapy dosing by scaling drug amounts to body size (e.g., mg/m²), which enhances efficacy and safety for cytotoxic agents by achieving more uniform plasma concentrations and mitigating toxicity risks associated with under- or overdosing.^[5]

History

Early Developments

The concept of total body surface area (TBSA) emerged in the late 19th century as physiologists sought to understand how body size influences physiological processes, particularly those related to heat dissipation and metabolic scaling. Karl M. Meeh, a German physiologist, introduced the first quantitative approach to estimating TBSA in 1879 through his seminal work on surface measurements of the human body. Meeh's motivation stemmed from observations in animal physiology, where body surface area was recognized as a key factor in heat loss and energy expenditure, prompting him to extend these principles to humans for more accurate modeling of basal metabolism and thermoregulation.^[5] Meeh derived his formula using direct empirical measurements on a limited sample of 6 adults and 10 children, approximating the body as geometric shapes to calculate surface areas. He proposed a weight-based equation, TBSA = 0.1053 \times weight^{2/3}, where TBSA is in square meters and weight is in kilograms; this allometric scaling with an exponent of approximately 2/3 reflected the geometric similarity principle that surface area grows disproportionately to volume as body size increases. These foundational body surface area (BSA) estimations later informed TBSA percentage methods in burn assessment.^[10] Early methods for TBSA assessment relied on labor-intensive direct techniques, such as creating plaster casts of body segments to unfold and measure their surface areas planimetrically, or using geometric approximations like cylinders and spheres to model limbs and torso. These techniques, while imprecise due to challenges in capturing body contours and individual variations, provided the foundational data for Meeh's formula and highlighted the need for non-invasive alternatives. Over time, these weight-only models evolved into more refined height-weight based formulas to better account for body proportions.^[9]

Key Contributors and Formulas

In the early 20th century, the estimation of total body surface area (TBSA) progressed beyond rudimentary weight-based approximations through the collaborative efforts of Delafield Du Bois, a mathematician, and her cousin Eugene F. Du Bois, a physician specializing in metabolism. In 1916, they introduced a seminal formula that integrated both height and weight for improved accuracy, derived from measurements of nine subjects using plaster molds and photographic paper to map body contours. This formula, BSA = 0.007184 \times W^{0.425} \times H^{0.725} (where W is weight in kilograms and H is height in centimeters), provided a more reliable predictor for adults and became a cornerstone for clinical applications in calorimetry and physiology.^[11] Building on this foundation, Eugene F. Du Bois extended the practical use of TBSA calculations in the 1920s through his research on basal metabolic rates, where precise surface area estimates were essential for normalizing energy expenditure data across diverse patient populations. His studies, including those published in the Archives of Internal Medicine, refined the methodological application of the 1916 formula by incorporating empirical validations from clinical observations, enhancing its utility in metabolic assessments without altering the core equation. This work underscored the limitations of prior weight-only approaches, such as the Meeh formula, and promoted multidimensional models for broader applicability.^[12] The evolution toward multidimensional TBSA models continued in the 1930s with Edith Boyd's contributions, which emphasized adaptations for age and sex to address variations across the lifespan. In her 1935 monograph, Boyd analyzed extensive anthropometric data from over 1,000 individuals, developing age-specific equations that adjusted surface area estimates for growth patterns and subtle sex differences in body proportions. Her self-adjusting formula, such as BSA = 0.03330 \times W^{(0.6157 - 0.0188 \log_{10} W)} \times H^{0.3} (where W is weight in grams and H is height in centimeters), particularly benefited pediatric calculations by accounting for disproportionate body segment changes during development.^[13]

Calculation Methods

Du Bois Formula

The Du Bois formula represents the most widely used mathematical method for estimating total body surface area (TBSA) based on an individual's height and weight, providing a precise calculation suitable for clinical applications in adults. Developed as an empirical model, it expresses TBSA in square meters (m²) using the equation:

\text{BSA (m²)} = 0.007184 \times \text{weight (kg)}^{0.425} \times \text{height (cm)}^{0.725}

This formula incorporates allometric exponents derived from logarithmic regression to account for the non-linear relationship between body dimensions and surface area.^[11]^[5] The formula was derived from direct measurements using plaster molds of nine Caucasian subjects (eight adults and one child) in 1916, where the molds were cut into strips to quantify the surface areas of body segments before applying statistical fitting to height and weight data. These measurements established a baseline that has been validated primarily for adult populations through subsequent comparisons with direct measurement techniques and its consistent performance in metabolic and dosing studies.^[11]^[14]^[5]^[15] To illustrate its application, consider an adult weighing 70 kg and measuring 170 cm in height. The calculation proceeds step by step as follows:

Raise the weight to the power of 0.425: $70^{0.425} \approx 6.08$ .
Raise the height to the power of 0.725: $170^{0.725} \approx 41.57$ .
Multiply the results: $6.08 \times 41.57 \approx 252.8$ .
Multiply by the constant: $0.007184 \times 252.8 \approx 1.81$ m².

This yields an estimated TBSA of approximately 1.81 m², a value typical for an average adult male of these dimensions.^[5]^[16] The Du Bois formula offers high accuracy for normal-weight adults, with errors typically under 5% when compared to direct measurements in validation studies for that population, though it overestimates in obese individuals. It remains widely used in many clinical protocols for applications such as chemotherapy dosing and burn assessment due to its empirical robustness and widespread adoption.^[5]

Mosteller Formula

The Mosteller formula provides a simplified method for estimating total body surface area (TBSA), particularly suited for rapid clinical calculations. Proposed by Robert D. Mosteller in 1987, it was developed to offer an easier-to-compute alternative to earlier models while maintaining comparable predictive accuracy for adults.^[17] The formula is expressed as:

\text{BSA (m²)} = \sqrt{\frac{\text{height (cm)} \times \text{weight (kg)}}{3600}}

BSA (m²)=3600height (cm)×weight (kg)

^[17] This equation requires only basic arithmetic operations—a multiplication, division, and square root—eliminating the need for complex exponents or additional constants found in traditional formulas. Its conciseness makes it particularly advantageous in resource-limited settings, such as field medicine or environments without access to calculators or software.^[18] It demonstrates accuracy equivalent to the Du Bois formula across typical adult body sizes, though minor discrepancies may arise at extremes of height and weight.^[19] To illustrate its application, consider an individual who is 170 cm tall and weighs 70 kg. First, multiply height by weight: 170 × 70 = 11,900. Next, divide by 3600: 11,900 / 3600 ≈ 3.3056. Finally, take the square root: √3.3056 ≈ 1.82 m². This process can be performed mentally or with minimal tools, highlighting the formula's practicality.^[17]

Burn Estimation Techniques

Rule of Nines

The Rule of Nines is a rapid, visual estimation technique used in emergency medicine to approximate the percentage of total body surface area (TBSA) affected by burns in adults, facilitating immediate triage and fluid resuscitation decisions.^[1] It divides the adult body into anatomical sections, each representing 9% or multiples of 9% of the TBSA, allowing clinicians to quickly sum the burned areas without precise measurements.^[1]

Body Region	Percentage of TBSA
Head and neck	9%
Each upper extremity	9%
Anterior trunk	18%
Posterior trunk	18%
Each lower extremity	18%
Perineum	1%

This division accounts for the anterior and posterior aspects of the torso and extremities symmetrically, with the total equaling 100%.^[1] For partial involvement of a section, such as a burn covering half of an arm, the percentage is proportionally adjusted (e.g., 4.5% for half an arm); smaller or irregular burns may be estimated using the patient's palm surface, which approximates 0.8-1% of TBSA.^[1] To apply the method, providers visually inspect the patient, identify partial- and full-thickness burns (excluding superficial ones), assign percentages to fully affected sections, adjust for partial areas, and sum the totals to determine the overall burned TBSA.^[1] The technique, introduced by Pulaski and Tennison in 1949 and refined by Scottish plastic surgeon Alexander Burns Wallace, was published in 1951 as a simplified tool for emergency burn assessment and management during the exposure treatment era.^[20]^[1] Wallace's approach built on earlier ideas but standardized the 9% multiples for practical use in acute settings, where speed is critical for initiating treatments like the Parkland formula for fluid replacement.^[20]^[1] While effective for adults in rapid triage, the Rule of Nines has limitations, including reduced accuracy in children due to their proportionally larger heads and smaller limbs relative to adults, potentially leading to overestimation of burned area in pediatric cases.^[1] It also introduces subjectivity in partial-section estimates and can overestimate TBSA in obese patients, where trunk proportions may exceed 36%.^[1] For more precise assessments, especially in non-emergency contexts, detailed charts like the Lund-Browder method are preferred.^[1]

Lund-Browder Chart

The Lund-Browder chart is an illustrated tool designed for estimating the total body surface area (TBSA) affected by burns, particularly valued for its age-specific adjustments that account for proportional changes in body regions during growth. Developed by Charles C. Lund and Norman C. Browder in 1944, it was based on anatomical studies of body surface area proportions across different ages, drawing from prior work on pediatric growth patterns to create a more precise method than earlier uniform approaches.^[3] The chart divides the body into specific regions—typically the head, neck, anterior and posterior trunk, genitalia, upper and lower limbs (bilateral), buttocks, and feet—and assigns percentage values to each that vary by age group, such as infants under 1 year, children aged 1–4 years, 5–9 years, 10–14 years, and adults over 15 years. For instance, the head constitutes 19% of TBSA in infants under 1 year but decreases to 9% in adults, while the lower limbs increase proportionally from smaller percentages in young children to larger shares in older individuals, reflecting developmental shifts. These adjustments ensure that estimations reflect the relative sizes of body parts more accurately across the lifespan.^[3] To use the chart, clinicians shade the burned areas on front and back body diagrams provided in the tool, then refer to an accompanying table to sum the corresponding percentages for the affected regions, excluding superficial first-degree burns. This process allows for detailed visualization of irregular burn patterns and rapid calculation of TBSA, often completed in clinical settings like emergency departments or burn units.^[3] Compared to simpler methods like the Rule of Nines, the Lund-Browder chart offers superior accuracy, especially for pediatric patients and irregularly shaped burns, by incorporating age-based refinements that reduce estimation errors and improve fluid resuscitation and treatment planning. Its widespread adoption stems from these advantages, making it the preferred standard in burn care guidelines.^[3]^[21]

Applications

Burn Assessment

Total body surface area (TBSA) estimation plays a central role in classifying burn severity, guiding immediate treatment decisions, and predicting patient outcomes in burn care. Burns are typically categorized based on the percentage of TBSA affected by partial- or full-thickness injuries, with superficial (first-degree) burns excluded from calculations as they do not require the same level of intervention. Burns are classified as minor if involving less than 15% TBSA in adults or 10% in children, often manageable on an outpatient basis; moderate if 15-20% TBSA in adults or 10-15% in children, typically requiring hospital admission for monitoring and supportive care; and major if exceeding 25% TBSA in adults or 20% in children, necessitating transfer to a verified burn center due to risks of systemic complications like hypovolemic shock and infection. Burn center referral is also recommended for partial- or full-thickness burns greater than 10% TBSA, or smaller burns in critical areas such as the face, hands, feet, genitalia, or major joints.^[22] A key application of TBSA assessment is in fluid resuscitation protocols, which are critical for preventing organ failure in large burns. The Parkland formula, a widely adopted guideline, calculates the initial 24-hour fluid requirement as 4 mL of lactated Ringer's solution per kilogram of body weight per percentage of TBSA burned, with half administered in the first 8 hours post-injury and the remainder over the next 16 hours.^[23] This approach addresses the profound capillary leak and third-space fluid shifts that occur in burns greater than 20% TBSA in adults or 10% in children, ensuring hemodynamic stability while avoiding over-resuscitation.^[23] TBSA estimation is integrated with burn depth assessment to provide a comprehensive injury grading that influences treatment planning. Partial-thickness (second-degree) burns involve the epidermis and part of the dermis, often healing with conservative measures, whereas full-thickness (third-degree) burns extend through the entire dermis and require surgical intervention like excision and grafting; combining these with TBSA percentage helps prioritize cases for escharotomy or intensive wound care.^[22] Accurate depth evaluation, typically via clinical examination or tools like laser Doppler imaging, refines the TBSA calculation to focus only on deeper injuries, optimizing resource allocation in trauma settings. Prognostic tools further leverage TBSA to forecast mortality and inform end-of-life discussions. The Baux score, a simple yet validated metric, sums the patient's age in years with the percentage of TBSA burned (revised version adds 17 points for inhalation injury), yielding a probability of survival; for example, scores above 100 often indicate high mortality risk, particularly in elderly patients.^[24] This score aids in stratifying patients for aggressive versus palliative care, with studies confirming its reliability across diverse burn populations.^[24]

Drug Dosing

In oncology, drug dosing is frequently normalized to total body surface area (TBSA), expressed in mg/m², to adjust for variations in patient body size and minimize toxicity risks, especially in larger individuals where fixed weight-based dosing could lead to excessive exposure.^[25] For instance, doxorubicin, an anthracycline used in regimens for breast cancer and lymphomas, is standardly administered at 60 to 75 mg/m² every three weeks to balance efficacy and cardiotoxicity.^[26] The practice of using TBSA for antineoplastic agents originated in the 1950s, with Donald Pinkel's seminal 1958 paper advocating its adoption over body weight alone, as TBSA more accurately predicts drug clearance by correlating better with physiological parameters such as cardiac output and renal function.^[27] This approach was extrapolated from animal studies to human trials, aiming to standardize doses across diverse body types in early chemotherapy protocols.^[28] Contemporary guidelines from the American Society of Clinical Oncology (ASCO) endorse TBSA-guided dosing protocols, recommending the use of actual body measurements without arbitrary caps (e.g., at 2 m²) for obese patients to ensure full therapeutic benefit, with calculations typically employing the Du Bois formula for its reliability in adult populations.^[29]^[5] Evidence supports that TBSA-based dosing reduces inter-patient pharmacokinetic variability for agents with narrow therapeutic indices, such as certain alkylating agents and antimetabolites, by achieving more consistent plasma exposure and lowering the incidence of severe adverse events compared to unadjusted methods.^[30] This normalization enhances safety and efficacy in elective settings, though ongoing research evaluates alternatives like fixed dosing for specific monoclonal antibodies.^[31]

Limitations and Variations

Accuracy and Errors

Common sources of inaccuracy in total body surface area (TBSA) calculations arise from the assumptions embedded in standard formulas and charts, which often fail to account for deviations from ideal body composition. For instance, in obese patients, methods such as the Rule of Nines tend to overestimate the surface area of extremities while underestimating the trunk and legs, with errors increasing as body mass index (BMI) rises due to the disproportionate enlargement of truncal areas relative to limbs.^[32] Similarly, in patients with amputations, conventional estimation techniques overestimate the total available body surface by not adjusting for the lost area of the amputated limb, thereby underestimating the percentage of TBSA affected by burns on the remaining body.^[33] Validation studies comparing traditional TBSA formulas to direct measurement techniques, such as three-dimensional (3D) body scanning, have revealed substantial discrepancies, with differences of up to approximately 20% between various formula-based estimates and smaller but significant variances when compared to 3D-derived values (e.g., 1.3% underestimation by advanced 3D tools relative to conventional methods).^[34]^[35] These investigations highlight the limitations of two-dimensional models that do not capture individual variations in body geometry or projection distortions. Such variances are particularly pronounced in non-standard body types, where empirical formulas like those detailed in prior sections on calculation methods exhibit reduced reliability against gold-standard imaging.^[34] The clinical consequences of these inaccuracies are profound, particularly in burn management where TBSA directly informs fluid resuscitation protocols. Overestimation can result in excessive fluid administration, leading to complications such as pulmonary edema and compartment syndrome, while underestimation risks inadequate resuscitation, potentially causing circulatory collapse, burn shock, and acute renal failure due to hypoperfusion.^[36]^[37] In severe cases, these errors contribute to worsened morbidity and mortality, underscoring the need for precise assessment to optimize outcomes.^[36] To mitigate these errors, advanced imaging technologies and software have been employed in research settings for more accurate TBSA measurements. Tools like BurnCase 3D utilize 3D scanning to generate patient-specific models, reducing overestimation rates to as low as 0.4% in pediatric cases and 1.5-2.8% in adults by accounting for body curvature and individual physiology, thereby minimizing inter-rater variability inherent in manual methods.^[38] These approaches enable interactive adjustments, offering a high-precision alternative that enhances reliability in complex scenarios.^[38]

Population-Specific Adjustments

In pediatric populations, total body surface area (TBSA) calculations require adjustments due to disproportionate body segment sizes compared to adults, particularly the larger head-to-body ratio that can constitute up to 19% of TBSA in infants versus 7% in adults.^[1] This necessitates age-specific methods, such as the Boyd formula, which was developed from growth data across infancy to adulthood and provides more accurate estimates for children by incorporating weight and height in a manner sensitive to developmental changes. For burn assessment, age-scaled charts like the Lund-Browder method adjust proportional areas (e.g., reducing head percentage with age) to improve precision over adult-oriented tools.^[1] Ethnic variations influence TBSA estimates, as formulas derived from Western (primarily Caucasian) populations often overestimate values in Asian groups due to differences in body build and limb lengths. Studies indicate Western formulas like DuBois can overestimate BSA by 3-5% in Japanese adults, prompting ethnicity-specific equations such as the Fujimoto formula, which yields 0.7-4.8% lower values for typical heights and weights.^[39] In Indian populations, a seminal 1958 study measured average BSA using direct cloth coverage and found that while the DuBois formula provided close approximations, body segment proportions (e.g., head at 5.8% versus higher in Western standards) differed significantly, highlighting potential value in regional adjustments.^[40] For obesity, defined as BMI >30 kg/m², weight-inclusive formulas like DuBois and Mosteller tend to inflate TBSA estimates by incorporating excess adipose tissue, potentially leading to overdosing in applications like chemotherapy.^[41] Adjustments commonly involve substituting ideal body weight or applying a corrected BSA equation for BMI >30, such as capping at 2 m².^[41] Sex differences in TBSA are minimal in standard calculations, as common formulas do not account for sex and yield similar values for individuals of the same height and weight, though population-level variations exist.^[42]

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