Will a Comprehensive Metabolic Panel (CMP) or Basic Metabolic Panel (BMP) show abnormalities in someone with acidosis?

Will a CMP or BMP Show Abnormalities in Acidosis?

Yes, a CMP or BMP will typically show abnormalities in someone with acidosis, most commonly manifesting as a low bicarbonate (CO2) level, though the specific pattern depends on whether the acidosis is metabolic or respiratory in origin.

Understanding What the CMP/BMP Measures

• The "CO2" reported on a CMP or BMP actually reflects total serum CO2 content, which is predominantly bicarbonate (70-85%), not arterial PCO2 1
• Normal serum bicarbonate range is 22-26 mmol/L, though more recent evidence suggests 23-30 mEq/L to avoid missing acid-base disorders 1
• Low serum bicarbonate concentrations (<22 mmol/L) almost always indicate metabolic acidosis 1

Metabolic Acidosis: What You'll See

In metabolic acidosis, the CMP/BMP will show a decreased bicarbonate level (<22 mmol/L), which is the hallmark finding 1, 2

• Metabolic acidosis is characterized by primary reduction in serum bicarbonate, associated with blood pH <7.35 1, 2
• The body attempts to compensate by increasing ventilation to eliminate CO2, resulting in a secondary decrease in arterial PCO2 of approximately 1 mmHg for every 1 mmol/L fall in serum bicarbonate 2
• Additional abnormalities may include elevated anion gap (calculated as [Na+] - [HCO3- + Cl-]), though this requires separate calculation and is not directly reported on the panel 2, 3

Key Clinical Point on Anion Gap

• Calculate the anion gap to determine the mechanism: normal anion gap (hyperchloremic) acidosis suggests bicarbonate loss (e.g., diarrhea), while elevated anion gap suggests accumulation of unmeasured acids (e.g., lactate, ketoacids) 2, 3
• Adjust the anion gap for hypoalbuminemia (extremely common in critically ill patients—96% have low albumin), as failure to do so will miss hidden gap acidosis in the majority of cases 4

Respiratory Acidosis: A Different Pattern

In chronic respiratory acidosis, the CMP/BMP paradoxically shows ELEVATED bicarbonate as a compensatory mechanism, not as the primary disorder 1

• The kidneys retain bicarbonate to buffer the acidity caused by chronically elevated CO2 1
• This elevated bicarbonate represents renal compensation for chronic CO2 retention, not metabolic alkalosis 1
• You cannot distinguish primary metabolic alkalosis from compensatory response to chronic respiratory acidosis using CMP/BMP alone—you need an arterial blood gas (ABG) to determine pH and PaCO2 1

When the CMP/BMP May Appear Normal Despite Acidosis

In approximately one-sixth of critically ill patients, base excess and plasma bicarbonate appear normal on routine testing, yet simultaneous acidifying and alkalinizing disturbances are present 4

• Multiple underlying mechanisms are present in most ICU patients with metabolic acidosis: unmeasured strong anions (98%), hyperchloremia (80%), and elevated lactate (62%) 5
• Hypoalbuminemia confounds interpretation—the almost ubiquitous low albumin in critically ill patients masks the true acid-base status 4

Critical Pitfall to Avoid

• Never rely solely on bicarbonate level when evaluating complex or critically ill patients—calculate the albumin-corrected anion gap and consider obtaining an ABG for complete assessment 4, 5
• The conventional anion gap detected hidden gap acidosis in only 31% of samples where it was actually present; when adjusted for hypoalbuminemia, detection improved dramatically 4

Specific Electrolyte Abnormalities You May See

Beyond bicarbonate, other CMP/BMP abnormalities commonly associated with acidosis include:

• Hyperkalemia: Acidosis causes potassium to shift from intracellular to extracellular space 2
• Elevated chloride: In normal anion gap (hyperchloremic) acidosis from bicarbonate loss 2, 3
• Elevated creatinine: When acidosis is due to renal failure with impaired acid excretion 1
• Low sodium: May occur with certain causes of metabolic acidosis 5

When to Obtain Additional Testing

Order an arterial blood gas (ABG) in these situations 1:

• Bicarbonate <18 mmol/L (severe metabolic acidosis requiring urgent evaluation)
• Bicarbonate >35 mmol/L during diuresis (to distinguish primary vs. compensatory alkalosis)
• Patient has respiratory symptoms or known COPD, obesity hypoventilation syndrome, or neuromuscular disease
• Complex or critically ill patients where the CMP/BMP pattern doesn't fit the clinical picture

Clinical Management Implications

• Bicarbonate <18 mmol/L indicates metabolic acidosis requiring pharmacological treatment and warrants consideration for hospitalization 1, 6
• Target serum bicarbonate ≥22 mmol/L in patients with chronic kidney disease to prevent complications including protein catabolism, bone disease, and CKD progression 1, 6
• In chronic kidney disease, measure serum bicarbonate monthly to maintain levels at or above 22 mmol/L 1, 6