Should Body Mass Index (BMI) codes be used as primary or secondary codes when coding?

BMI Coding in Medical Records: Secondary Diagnosis

BMI codes should be used as secondary diagnosis codes, not primary codes, when documenting patient obesity status in medical records. 1

Rationale for BMI as Secondary Code

• BMI is a measurement tool used to estimate body fat and quantify associated health risks, but it is not a disease or condition in itself 1
• BMI serves as a clinical indicator that helps identify patients at increased risk of future morbidity and mortality 1
• BMI documentation supports the diagnosis of obesity and related conditions, which would be coded as the primary diagnoses 1
• BMI is considered a vital health parameter that should be routinely measured and documented, similar to other vital signs, but represents a measurement rather than a primary diagnosis 2

Proper Documentation of BMI

• BMI should be calculated and documented in the medical record at annual visits or more frequently when clinically indicated 1
• BMI is calculated as weight in kilograms divided by the square of height in meters (kg/m²) 1
• For proper coding, both the calculated BMI value and the associated weight status (overweight, obesity class I, II, or III) should be documented 1
• When BMI exceeds the capacity of available scales (>350 lb), BMI should be considered >40 kg/m² for documentation purposes 1

Clinical Classification Based on BMI

• Standard BMI classifications include: overweight (BMI 25-29.9 kg/m²), obesity class I (BMI 30-34.9 kg/m²), obesity class II (BMI 35-39.9 kg/m²), and obesity class III (BMI ≥40 kg/m²) 1
• For Asian populations, including Asian Americans, lower BMI cutoff points should be used to define overweight and obesity due to differences in body composition and cardiometabolic risk 1, 3
• Class IV obesity (BMI >50 kg/m²) and Class V obesity (BMI >60 kg/m²) represent additional classifications for severe obesity that should be documented when present 4

Importance of Accurate BMI Documentation

• Despite the clinical importance of obesity, studies show it is significantly under-documented in medical records, with one study finding only 5.6% of patients with BMI ≥30 had obesity documented in their problem list 5
• Proper documentation of BMI is essential for:
  • Identifying patients who would benefit from weight management interventions 1
  • Supporting medical necessity for obesity-related treatments 1
  • Enabling accurate population health management and research 6
  • Facilitating appropriate reimbursement for obesity-related care 6

Limitations of BMI and Additional Measurements

• BMI alone may not accurately reflect body composition in certain populations, including muscular individuals, older adults, and certain ethnic groups 1, 7
• For more comprehensive assessment, waist circumference should be measured alongside BMI, especially in patients with BMI <35 kg/m² 1
• Elevated waist circumference (≥40 inches in men, ≥35 inches in women) indicates increased cardiometabolic risk even at lower BMI levels 1
• Body fat percentage may be a better predictor of mortality risk than BMI, particularly in younger adults 8

Common Pitfalls in BMI Coding

• Using BMI as a primary diagnosis rather than as a supporting measurement for conditions like obesity, metabolic syndrome, or diabetes 6
• Failing to document BMI when it falls outside normal range 2, 5
• Not considering ethnic-specific BMI thresholds, particularly for Asian populations 3
• Relying solely on BMI without additional measurements like waist circumference 1, 7

BMI documentation is essential for comprehensive patient care but should be coded as a secondary diagnosis that supports primary diagnoses such as obesity or related conditions.