The 9% rule is a guideline used by medical professionals to estimate the total body surface area (TBSA) affected by a burn injury. It states that in adults, the palm of the patient’s hand (excluding fingers and thumb) equates to approximately 1% of their TBSA. This allows healthcare providers to quickly assess the extent and severity of burn injuries.
How does the rule work?
To use the 9% rule, the palm of the patient’s hand is placed over the area of the burn. The number of “palm-sized” areas that fit into the burnt region is counted, with each palm area equivalent to around 1% TBSA. For example, if the burnt area covers 3 palm-sized patches, this suggests a 3% TBSA burn.
The origin of the 9% value comes from the approximate ratio of the palm area to the entire adult body surface area. An average adult’s palm, excluding fingers and thumb, represents around 0.8% to 1% of their TBSA. Rounding this span to 1% provides the basis of the 9% rule.
Guidelines for using the 9% rule
When utilizing the 9% rule, medical professionals should follow these guidelines:
- Use the patient’s own hand for comparison, not the assessor’s hand, as palm sizes vary.
- Press the palm fully flat over the burnt area to get an accurate measurement.
- Assess both the palmar (inner) and dorsal (outer) hand surface areas.
- Include areas where skin has been lost or contracted from burn injury.
- Recalculate regularly as swelling from burns can expand affected areas.
Limitations
While useful for initial evaluation, the 9% rule has limitations including:
- Doesn’t account for variability in palm size based on a person’s age, sex, and body size.
- Can over or underestimate TBSA, especially in severe burns.
- Difficult to assess irregular, scattered burns.
- Less accurate in infants and children due to proportionally larger palm surface area.
When accuracy matters
In cases where an accurate TBSA percentage is critical for gauging treatment needs, the 9% rule should be followed by a more precise assessment using refined measurement tools such as burn charts, computer software, or tracing/photographic techniques.
Using the Lund and Browder chart
The Lund and Browder chart improves on the 9% rule by providing a pictorial diagram of body regions with detailed TBSA percentages tailored to different ages. This supports more precise calculation of burn surface area and depth.
To use the chart, the assessor shades the burnt regions on the diagram, then calculates the percentages assigned to each shaded area. This method accounts for variations in body proportions across ages, unlike the 9% rule’s one-size-fits-all approach.
Advantages of the Lund and Browder chart
- More accurate TBSA assessment than the 9% rule.
- Adjusts for patient age and development.
- Clearly visualizes injury distribution.
- Allows calculation of partial/mixed depth burns.
Limitations of the Lund and Browder chart
- More time consuming than the 9% rule.
- Requires pictorial diagrams and calculation.
- Can still over or underestimate TBSA.
- Difficult with circumferential burns.
When to estimate TBSA
Quick TBSA estimation using the 9% rule is often needed in emergency situations to prioritize care. However, a proper in-depth assessment should follow when feasible. Estimating TBSA is important for:
- Determining intravenous fluid resuscitation needs.
- Guiding whether to transfer patients to a specialist burn unit.
- Evaluating prognosis and mortality risk.
- Planning wound care requirements.
- Predicting the donor skin available for grafting.
TBSA thresholds for referral
Patients with extensive burns requiring specialized care should be transferred to a burn center. Some common TBSA thresholds for referral include:
| Age | TBSA threshold for referral |
|---|---|
| Adults (ages 13-50) | >20% TBSA partial thickness burns |
| Older children (ages 5-13) | >10% TBSA partial thickness burns |
| Younger children ( | >5% TBSA partial thickness burns |
| All ages | Full thickness burns >5% TBSA |
| All ages | Burns involving face, hands, feet, genitals |
| All ages | Electrical or chemical burns |
| All ages | Inhalation injury or requiring mechanical ventilation |
Estimating burn depth
In addition to surface area, assessing the depth of a burn is also important for directing treatment. Burn depths are classed as:
- Superficial (1st degree) – Epidermal skin layer only, redness and pain.
- Partial thickness (2nd degree) – Extends into dermis, blistering.
- Full thickness (3rd degree) – Entire thickness of skin affected, charring.
Burn depth guides decisions on debridement, dressings, need for grafting, and scarring risk. Superficial burns may heal without grafting, while deep dermal or full thickness burns often require excision and skin grafts.
Calculating the burn surface area
Once the TBSA percentage is estimated, the actual burn surface area in square meters can be calculated using the ‘Rule of Nines’. This converts the TBSA to body surface area (BSA) in meters squared.
In adults, the formula is:
BSA (m2) = ([%TBSA x Weight (kg)] / 90) x Height (cm) / 100
For example, for a 70kg adult measuring 170cm tall with 18% TBSA burns:
BSA = ([18 x 70kg] / 90) x 170cm / 100
= 10.92 m2
Knowing the BSA of the burnt region helps calculate the fluid volume for resuscitation.
Fluid calculation and resuscitation
Burn injuries cause extensive fluid losses which must be replaced urgently to maintain circulation. Formulae are available to guide the rate and volume of intravenous fluid replacement needed.
The classic Parkland formula uses the BSA to calculate fluid rates. It prescribes 4ml/kg/%TBSA of lactated Ringer’s solution to be given over 24 hours, with half infused in the first 8 hours post-burn.
Using the earlier example of an 18% TBSA burn, the Parkland formula gives:
4ml x 70kg x 18% TBSA = 5,040ml total fluid
2,520ml over first 8 hours
420ml/hour for the first 8 hours
This initial calculation helps guide urgent fluid resuscitation, though additional fluid adjustments are often needed based on close monitoring of urine output and other parameters.
Limitations of burn surface area estimation
While measuring burnt surface area is vital for directing treatment, limitations exist including:
- Accuracy reduced in severe and complex burns.
- Swelling can enlarge affected areas over time.
- Doesn’t account for individual factors like age, comorbidities.
- Can underestimate deeper tissue damage below skin.
Clinical judgement combined with these calculations provides the best approach. Assessments should be repeated regularly and management adapted accordingly.
When to use more advanced techniques
For major burns where precision is essential, several techniques offer improved accuracy over basic TBSA estimation:
- Computer programs – Calculate area from tracings or 3D scans.
- Transparent film – Burn outlines traced on films overlaid on body diagrams.
- Photography – Wound analysis software applied to digital photographs.
- Laser Doppler Imaging – Scans blood flow to assess burn depth.
However, these advanced options may not be readily available in emergencies or small healthcare facilities.
Conclusion
The 9% rule provides a quick and pragmatic way for first responders to estimate TBSA in acute burns, guiding critical management decisions. However, its limitations mean more accurate techniques should be applied when feasible. Calculating TBSA remains central to gauging severity, prioritizing care, calculating fluid needs, and planning surgery. Ongoing re-evaluation and a personalized approach are key to optimizing burn patient outcomes.