USMLE® Step 1 Question of the Day: Rule of nines

USMLE® Step 1 Question of the Day: Rule of nines

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The Rule of Nines is a method used in emergency departments to quickly estimate the extent of burns on a patient’s body. This USMLE® Step 1 Question of the Day will test your knowledge on this topic.

A 25-year-old man is brought to the emergency department with severe burns on his body sustained in a house fire about an hour ago. His left lower leg was trapped under a burning beam, and as a result, he could not immediately escape from the house. Temperature is 38.5°C (101.3°F), pulse is 120/min, respirations are 20/min, and blood pressure is 105/55 mmHg. He is conscious but is moaning in pain. There is no singed hair on his face or nostrils, and he is breathing spontaneously. Physical examination shows 2nd-degree burns to the anterior chest, abdomen, and entire left leg. Which of the following most likely represents the total percentage of his whole body surface area involved in the burn according to the rule of nines?

A. 27%

B. 36%

C. 45%

D. 54%

E. 63%

Scroll down for the correct answer!

The correct answer to today’s USMLE® Step 1 Question is…

B. 36%

Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

A. 27%

Incorrect: Following the rule of 9’s, this patient’s anterior chest (9% of TBSA) and abdomen (9% of TBSA), together with anterior (9% of TBSA) and posterior (9% of TBSA) parts of the left leg, constitute 36% involvement of the TBSA (9+9+9+9=36%)

C. 45%

Incorrect:  Following the rule of 9’s, this patient’s anterior chest (9% of TBSA) and abdomen (9% of TBSA), together with anterior (9% of TBSA) and posterior (9% of TBSA) parts of the left leg, constitute 36% involvement of the TBSA (9+9+9+9= 36%)

D. 54%

Incorrect: Following the rule of 9’s, this patient’s anterior chest (9% of TBSA) and abdomen (9% of TBSA), together with anterior (9% of TBSA) and posterior (9% of TBSA) parts of the left leg, constitute 36% involvement of the TBSA (9+9+9+9= 36%)

E. 63%

Incorrect: Following the rule of 9’s, this patient’s anterior chest (9% of TBSA) and abdomen (9% of TBSA), together with anterior (9% of TBSA) and posterior (9% of TBSA) parts of the left leg, constitute 36% involvement of the TBSA (9+9+9+9= 36%)

Main Explanation

This patient has sustained 2nd-degree burns on different parts of his body (anterior chest and abdomen, left leg) after being trapped in a house fire. His vitals are unstable (increased body temperature, tachycardia, hypotension) due to fluid loss through the damaged skin and widespread cytokine activation. He needs urgent medical attention to avoid possible complications (e.g., infection, lactic acidosis) and death.

Management of burn patients (e.g., fluid resuscitation, antibiotics) depends on the severity of the burns and the total body surface area (TBSA) involved. Different methods are employed to assess the TBSA of a patient with burns (e.g., Lund-Browder charting, Palm method, etc.). One such method is known as the “Rule of Nines.” It is one of the most expeditious methods to estimate TBSA in adults. It attributes 9% of the TBSA to each of the following areas: head, anterior chest, posterior chest, anterior abdomen, posterior abdomen, right arm, left arm, and anterior and posterior parts of the right and left legs each. The perineum is assigned 1% of the TBSA. Therefore in the above patient, his anterior chest (9%) and abdomen (9%), together with anterior (9%) and posterior (9%) parts of the left leg, constitute 36% (9+9+9+9) involvement of the TBSA.

wallace rule of nines estimated body surface are in children and adults

Major Takeaway

Burn management is complex and requires careful consideration of fluids. The “Rule of 9’s” can be used to estimate the total body surface area involved. This statistic is useful for calculating the total fluid resuscitation a burn patient will need in the first 24 hours following a burn injury.

References

  • Moore, R. A., Waheed, A., & Burns, B. (2019). Rule of Nines. In StatPearls [Internet]. StatPearls Publishing.
  • Monafo, W. W. (1996). Initial management of burns. New England Journal of Medicine, 335(21), 1581-1586.
  • Wachtel, T. L., Berry, C. C., Wachtel, E. E., & Frank, H. A. (2000). The inter-rater reliability of estimating the size of burns from various burn area chart drawings. Burns, 26(2), 156-170.

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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB. 


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