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Difficulty: MediumNephrology

A 35-year-old man is brought to the urgent care clinic by his roommate because of confusion and an inability to keep fluids down for the past 4 days. The roommate reports the patient has a history of type 1 diabetes mellitus but lost his insurance recently and started rationing his insulin. Over the last 96 hours, the patient has had severe nausea and vomited more than 10 times daily. He appears lethargic and is difficult to arouse. His temperature is 37.1°C (98.8°F), blood pressure is 92/56 mm Hg, pulse is 124/min, and respirations are 16/min. Examination shows deeply sunken eyes, dry oral mucosa, and delayed capillary refill.

Laboratory TestResult
Serum sodium138 mEq/L
Serum potassium2.9 mEq/L
Serum chloride80 mEq/L
Serum bicarbonate25 mEq/L
Blood urea nitrogen52 mg/dL
Serum creatinine2.1 mg/dL
Plasma glucose640 mg/dL
Arterial Blood Gas (Room Air)Result
pH7.42
pCO241 mm Hg
pO292 mm Hg

Analysis of these laboratory findings most accurately indicates which of the following underlying acid-base disturbances?

  1. A
    Mixed non-anion gap metabolic acidosis and metabolic alkalosis
  2. B
    Isolated high anion gap metabolic acidosis
  3. C
    High anion gap metabolic acidosis and non-anion gap metabolic acidosis
  4. Mixed high anion gap metabolic acidosis and metabolic alkalosisAnswer
  5. E
    Mixed high anion gap metabolic acidosis and respiratory alkalosis

Answer

Mixed high anion gap metabolic acidosis and metabolic alkalosis
The correct answer is mixed high anion gap metabolic acidosis (HAGMA) and metabolic alkalosis. Despite a normal pH (7.42) and bicarbonate (25 mEq/L), this patient's anion gap is markedly elevated at 33 mEq/L (138−(80+25)=33138 - (80 + 25) = 33). This indicates the presence of a HAGMA, which in this clinical context is diabetic ketoacidosis. To determine if another metabolic disorder is present, the delta-delta concept must be applied. The expected original bicarbonate is calculated by adding the delta anion gap (ΔAG=33−12=21\Delta AG = 33 - 12 = 21) to the measured bicarbonate: 25+21=4625 + 21 = 46 mEq/L. Because this value is much higher than the normal bicarbonate range (22-26 mEq/L), there is a concurrent metabolic alkalosis. Clinically, the HAGMA is driven by ketoacid accumulation, while the metabolic alkalosis is driven by profound gastric acid and chloride loss from intractable vomiting.

Step-by-Step Solution

1
Evaluate the primary acid-base status using pH, pCO2pCO_2, and HCO3−HCO_3^-.
The pH is 7.42 (normal), pCO2pCO_2 is 41 mm Hg (normal), and HCO3−HCO_3^- is 25 mEq/L (normal).
To identify any overt acidemia or alkalemia and primary respiratory or metabolic disturbances.
2
Calculate the serum anion gap (AG=Na+−(Cl−+HCO3−)AG = Na^+ - (Cl^- + HCO_3^-)).
AG=138−(80+25)=33AG = 138 - (80 + 25) = 33 mEq/L.
An anion gap must be calculated in all critically ill patients, regardless of a normal pH, to detect hidden metabolic acidosis.
3
Calculate the delta gap (ΔAG=MeasuredAG−NormalAG\Delta AG = Measured AG - Normal AG) and expected original bicarbonate.
ΔAG=33−12=21\Delta AG = 33 - 12 = 21 mEq/L. Expected original HCO3−=MeasuredHCO3−+ΔAG=25+21=46HCO_3^- = Measured HCO_3^- + \Delta AG = 25 + 21 = 46 mEq/L.
To determine if a second concurrent metabolic disturbance is present alongside the high anion gap metabolic acidosis.
4
Interpret the delta-delta result.
An expected original HCO3−HCO_3^- of 46 mEq/L is significantly greater than the normal range (22-26 mEq/L), indicating a concurrent metabolic alkalosis.
The large excess in bicarbonate points to a second process adding bicarbonate to the serum, consistent with the patient's severe vomiting.

Key Concept

Patients with a normal pH and bicarbonate can still have severe, opposing mixed acid-base disorders. Calculating the anion gap and delta-delta ratio is essential to uncover a hidden high anion gap metabolic acidosis that is masked by a concurrent metabolic alkalosis.

Alternative Method

Instead of calculating the expected original bicarbonate, the Δ\Delta Ratio can be used: ΔRatio=ΔAGΔHCO3−=33−1224−25=21−1\Delta Ratio = \frac{\Delta AG}{\Delta HCO_3^-} = \frac{33 - 12}{24 - 25} = \frac{21}{-1}. A negative denominator (because measured bicarbonate is higher than the normal baseline) or a ratio >2.0> 2.0 strongly indicates a concurrent metabolic alkalosis.
Estimated Time:1m 30s
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Nephrology

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