Final answer:
ICD-10-CM codes for burns are based on their location, severity, and the total body surface area affected. For the third-degree burns on the left leg and chest and the second-degree burns on the left arm, the codes would reflect each unique site and the TBSA percentages provided.
Step-by-step explanation:
The patient's burns can be classified and coded using the ICD-10-CM codes based on the description provided. For the third-degree burns of the anterior and posterior portions of the left leg, the ICD-10-CM code would reflect the site and the extent of the burns in terms of the total body surface area (TBSA) affected, which in this case is 12.5%. Similarly, for the third-degree burn on the anterior portion of the left chest, the ICD-10-CM code would be chosen for the affected body region with 4.5% TBSA. Lastly, for the second-degree burns on the left upper arm with 7% TBSA, an appropriate ICD-10-CM code would also be selected based on the burn degree and the specific location.
It is important to note that each code should also reflect the severity (degree) of the burn. The ICD-10-CM codes themselves are not provided in this response, as coding requires careful selection and sometimes clinical interpretation in addition to the 'rule of nines' to ensure the accurate coding of each burn site.