What is the appropriate diagnostic workup for a severely underweight adult with a BMI of 17 kg/m²?

Diagnostic Workup for BMI 17 kg/m²

A BMI of 17 kg/m² represents moderate thinness requiring systematic evaluation for underlying causes, nutritional deficiencies, and associated complications, with the workup guided by the ESPEN diagnostic criteria for malnutrition. 1

Initial Assessment Framework

Confirm Malnutrition Diagnosis

• Document BMI <18.5 kg/m² (this patient meets ESPEN Option 1 criteria for malnutrition diagnosis) 1
• Obtain detailed weight history: quantify any weight loss >5% over 3 months or >10% over any timeframe 1
• Assess recent food intake patterns and any reduction in oral intake 1

Essential History Components

Disease-Related Causes:

• Screen for malignancy (particularly gastrointestinal, lung, or hematologic cancers) 1
• Evaluate for chronic infections: tuberculosis, HIV, chronic hepatitis 2
• Assess for gastrointestinal disorders: malabsorption syndromes (celiac disease, inflammatory bowel disease, chronic pancreatitis), dysphagia, chronic diarrhea 1
• Review for endocrine disorders: hyperthyroidism, uncontrolled diabetes mellitus, adrenal insufficiency 1
• Evaluate psychiatric conditions: anorexia nervosa, depression, substance use disorders 1

Medication Review:

• Document all medications that may suppress appetite or cause weight loss 3
• Assess for drug-induced malabsorption 1

Functional Assessment:

• Quantify exercise capacity and activities of daily living (severely underweight patients have markedly reduced cardiorespiratory fitness) 1
• Document any orthopedic limitations affecting mobility 1

Physical Examination Priorities

Anthropometric Measurements:

• Measure mid-upper arm circumference (MUAC): values <20.5 cm in males or <18.5 cm in females indicate severe undernutrition 2
• Calculate fat-free mass index (FFMI): low FFMI is <15 kg/m² in females and <17 kg/m² in males 1
• Assess for signs of muscle wasting and subcutaneous fat loss 1

Clinical Signs of Deficiency:

• Examine for micronutrient deficiencies: pallor, glossitis, cheilosis, dermatitis, neuropathy 1
• Assess cardiovascular status: bradycardia, hypotension, signs of heart failure (obesity cardiomyopathy can paradoxically occur with chronic malnutrition) 1
• Evaluate for edema suggesting hypoalbuminemia 1

Laboratory Workup

Baseline Metabolic Panel:

• Complete blood count with differential (anemia, leukopenia) 1
• Comprehensive metabolic panel: electrolytes, renal function, liver enzymes, albumin 1
• Fasting glucose and HbA1c (diabetes can present with weight loss) 3
• Thyroid function tests (TSH, free T4) 1

Nutritional Markers:

• Serum albumin and prealbumin (though albumin is influenced by inflammation and hydration status) 1
• Vitamin D, B12, folate levels 1
• Iron studies: ferritin, transferrin saturation, total iron-binding capacity 1

Disease-Specific Testing Based on Clinical Suspicion:

• Celiac serology (tissue transglutaminase IgA with total IgA) 1
• HIV testing 2
• Inflammatory markers: C-reactive protein, erythrocyte sedimentation rate (chronic inflammation is a component of disease-related malnutrition) 1
• Fecal calprotectin or fecal elastase if malabsorption suspected 1

Imaging Studies

Chest Radiography:

• Evaluate for pulmonary tuberculosis, malignancy, or chronic lung disease 2
• Not routinely indicated in very low-risk patients but warranted given BMI 17 kg/m² 1

Additional Imaging as Indicated:

• CT chest/abdomen/pelvis if malignancy suspected based on history or laboratory abnormalities 1
• Upper endoscopy with duodenal biopsies if celiac disease or malabsorption suspected 1

Functional Assessment

Grip Strength Testing:

• Hand grip strength correlates with BMI and predicts functional capacity (positively associated with BMI in linear regression models) 2
• Reduced grip strength indicates sarcopenia and increased morbidity risk 2

Body Composition Analysis (if available):

• Bioelectrical impedance or DEXA scan to quantify fat-free mass and fat mass 3
• Particularly useful when FFMI calculation needed for ESPEN malnutrition criteria 1

Risk Stratification

Nutritional Risk Screening (NRS-2002):

• Apply validated NRS tool incorporating BMI <20.5 kg/m², weight loss >5% in 3 months, reduced food intake, and disease severity 1
• NRS predicts hospital complications and mortality in surgical and medical patients 1

Cardiovascular Risk Assessment:

• ECG to evaluate for bradycardia, QTc prolongation (refeeding syndrome risk) 1
• Consider echocardiography if cardiac symptoms present or prolonged malnutrition (assess for reduced cardiac mass and function) 1

Critical Pitfalls to Avoid

• Do not assume low BMI equals simple caloric insufficiency—disease-related malnutrition involves metabolic derangements and chronic inflammation requiring treatment of underlying conditions 1
• Recognize that BMI 17 kg/m² carries significant metabolic risk with increased surgical morbidity and mortality if procedures required 1
• Screen for refeeding syndrome risk before initiating aggressive nutritional support (check baseline phosphorus, magnesium, potassium) 1
• In older adults (>70 years), use BMI <20 kg/m² threshold rather than <18.5 kg/m² for malnutrition diagnosis 1

Immediate Management Considerations

While completing workup:

• Initiate nutritional counseling and document food intake 1
• Consider oral nutritional supplements if intake inadequate 1
• Address any identified reversible causes immediately 1
• Monitor electrolytes closely if initiating refeeding 1