In an elderly immobile patient with sacral pressure ulcers and labs showing serum bicarbonate 8 mEq/L, anion gap 18, blood urea nitrogen 36 mg/dL, creatinine 1.5 mg/dL, and white blood cell count 19 ×10⁹/L, what is the most likely diagnosis and what are the immediate management priorities?

Sepsis from Infected Sacral Pressure Ulcer with High Anion Gap Metabolic Acidosis and Acute Kidney Injury

This elderly immobile patient with sacral pressure ulcers presenting with severe high anion gap metabolic acidosis (bicarbonate 8 mEq/L, anion gap 18), leukocytosis (WBC 19), and acute kidney injury (BUN 36, creatinine 1.5) most likely has sepsis from an infected pressure ulcer, and requires immediate resuscitation, urgent surgical debridement, and broad-spectrum antibiotics covering polymicrobial flora including MRSA. 1, 2

Immediate Diagnostic Assessment

The clinical picture indicates sepsis with severe metabolic acidosis requiring urgent intervention:

• The combination of severe metabolic acidosis (bicarbonate 8 mEq/L), elevated anion gap (18 mmol/L, which exceeds the contemporary reference interval upper limit of 18 mmol/L), leukocytosis, and acute kidney injury in an immobile patient with sacral pressure ulcers strongly suggests sepsis from an infected pressure ulcer 3, 4

• The anion gap of 18 represents the upper limit of the modern reference interval (10-18 mmol/L), indicating accumulation of unmeasured anions from lactic acidosis secondary to sepsis 4

• Acute kidney injury (creatinine 1.5, BUN 36) contributes to both the acidosis and elevated anion gap through impaired acid excretion and uremic acid accumulation 5, 6

• Leukocytosis (WBC 19) combined with the clinical context confirms active systemic infection 2

Severity Assessment and Resuscitation Targets

This patient requires immediate aggressive resuscitation:

• Use SOFA or qSOFA scoring to assess sepsis severity and guide management intensity 3

• Target mean arterial pressure ≥65 mmHg, urine output ≥0.5 mL/kg/h, and lactate normalization during resuscitation 3

• Obtain serum lactate immediately, as lactic acidosis from tissue hypoperfusion is the most likely contributor to the elevated anion gap in septic patients 3, 5

• The profound acidosis (bicarbonate 8 mEq/L) requires rapid fluid resuscitation with crystalloids; initial bolus of at least 20 mL/kg should be administered 3

• Monitor hemodynamic response to fluid loading: look for ≥10% increase in systolic/mean arterial pressure, ≥10% reduction in heart rate, and improvement in mental status and peripheral perfusion 3

Antibiotic Management

Initiate broad-spectrum empiric antibiotics within 1 hour, covering the polymicrobial flora typical of infected pressure ulcers:

• First-line regimen: piperacillin-tazobactam PLUS vancomycin to cover Staphylococcus aureus (including MRSA), Enterococcus spp., gram-negative bacilli (E. coli, Proteus, Pseudomonas), and anaerobes (Bacteroides fragilis, Clostridium perfringens, Peptostreptococcus) 1

• Vancomycin is essential given the nursing home setting, which represents a high-risk environment for MRSA colonization 1

• Alternative regimen: carbapenem (meropenem or imipenem) alone provides adequate polymicrobial coverage including anaerobes 1

• For β-lactam allergy: fluoroquinolone (levofloxacin) plus metronidazole 1

• Pressure ulcer infections are invariably polymicrobial, involving aerobic gram-negative bacilli, gram-positive cocci, and anaerobes 3, 1

Surgical Intervention

Urgent surgical debridement is mandatory—antibiotics alone are insufficient:

• Surgical consultation should occur simultaneously with resuscitation and antibiotic administration 3, 1

• During debridement, obtain deep tissue or bone specimens for culture to guide targeted antimicrobial therapy 1, 2

• Deep tissue biopsy is the gold standard for microbiologic diagnosis; surface swabs cannot distinguish colonization from infection and should be avoided 3, 1, 2

• If osteomyelitis is suspected (exposed bone, probe-to-bone positive), obtain MRI (98% sensitive, 89% specific) and bone biopsy for definitive diagnosis 3, 2

Metabolic Acidosis Management

The severe metabolic acidosis requires supportive care focused on treating the underlying sepsis:

• Bicarbonate therapy is not routinely recommended for metabolic acidosis unless pH <6.9 3, 5, 6

• Treatment should focus on source control (surgical debridement), infection eradication (antibiotics), and restoration of tissue perfusion (fluid resuscitation) 3, 5

• The acidosis will improve with adequate resuscitation and treatment of sepsis; aggressive attempts to normalize pH with bicarbonate are controversial and may cause complications 5, 6

• Monitor venous pH and anion gap every 2-4 hours to assess response to therapy 3

Acute Kidney Injury Management

The elevated creatinine (1.5) and BUN (36) indicate acute kidney injury requiring specific attention:

• Ensure adequate fluid resuscitation to restore renal perfusion 3

• Avoid nephrotoxic agents when possible 3

• If the patient received vancomycin and creatinine continues to rise, consider switching to daptomycin, which is preferred in renal impairment 1

• Monitor urine output closely as a marker of adequate resuscitation (target ≥0.5 mL/kg/h) 3

Antibiotic Duration and Monitoring

Standard therapy duration is 2-4 weeks for moderate-to-severe soft tissue infections:

• Duration depends on completeness of debridement, presence of cellulitis, and clinical response 1

• Extend to 6 weeks if osteomyelitis is confirmed and surgical debridement with flap reconstruction performed 1

• Continue antibiotics until clinical signs resolve (reduced erythema, warmth, purulent drainage, systemic symptoms); therapy does not need to wait for complete wound healing 1

• If no improvement after 7 days, discontinue antibiotics for 2-3 days, repeat cultures, and initiate alternative regimen based on new results 1

• De-escalate to narrower spectrum once culture results available 1

Common Pitfalls to Avoid

• Do not obtain surface swab cultures—they are unreliable and cannot differentiate colonization from infection 3, 1, 2

• Do not delay surgical debridement—antibiotics alone will not adequately treat infected pressure ulcers 1

• Do not aggressively treat the acidosis with bicarbonate—focus on treating the underlying sepsis 5, 6

• Do not use linezolid empirically for this indication 1

• Blood cultures are not routinely indicated unless systemic infection or sepsis is present, which is clearly the case here 3, 2