Case 35 Endocrinology

Polyuria and Polydipsia

Work it one clue at a time. The diagnosis stays hidden until you reveal it.

Clue 1

18 M presents with polyuria, polydipsia, abdominal pain, and intense vomiting.

RR is 28 breaths per minute. ABG: pH is 7.29, PaCO2 is 26 mm Hg, HCO3− is 12.

What acid/base disturbance(s) is present and what is the underlying cause?

Clue 2

Measurement of the pH of blood (from ABG) is the first diagnostic step in determining the nature of an acid-base disturbance. Acidemia refers to pH < 7.35; alkalemia refers to pH > 7.45.

With a pH of 7.29, our patient is acidemic.

Clue 3

The suffix -emia refers to the acid-base state of blood (either acidic or alkaline), whereas the suffix -osis refers to system-based disorders that affect the acid-base state of blood.

Although there can only be one acid-base state of blood at a given time (either acidemia or alkalemia), multiple acid-base disorders can exist simultaneously.

Serum HCO3− (from the BMP or ABG) and PaCO2 (from the ABG) are necessary to identify the underlying disorder(s) involved in an acid-base disturbance.

Primary metabolic acidosis is characterized by the combination of acidemia and low serum HCO3−, as in this case (recall HCO3− is 12).

Clue 4

So our pt has a primary metabolic acidosis. What’s the next step?

The subtypes of metabolic acidosis are non–anion gap (non-AG) metabolic acidosis and anion gap (AG) metabolic acidosis.

We’ll need some more labs: Na+ 128 mEq/L, Cl− 82 mEq/L, HCO3− 12 mEq/L

To determine the type of metabolic acidosis (non-AG or AG), the AG must be calculated.

In this case, AG = 128 − (82 + 12) = 34 mEq/L, which is elevated (normal is ≤12 mEq/L).

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