The following results were obtained from a 32-year-old male:

Parameter

Patient Value

Normal Adult Range

Plasma

Sodium

138 mmol/L

135 – 145

Potassium

3.4 mmol/L

3.4 – 5.0

Chloride

118 mmol/L*

100 – 110

Bicarbonate

15 mmol/L*

22 – 27

Arterial Blood Gas

FiO2

0.3

pH

7.32*

7.35 – 7.45

PO2

125 mmHg (16.4 kPa)

PCO2

30 mmHg (4 kPa)*

35 – 45 (4.6 – 6.0)

Base Excess

-10 mmol/L*

-2 – +2

Urine

pH

5.0

4.6 – 8.0

Sodium

40 mmol/L

Potassium

10 mmol/L

Chloride

80 mmol/L

a) Describe the abnormalities on the blood investigations.

b) What is the underlying mechanism for the primary abnormality?

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College Answer

Answer

a)

A-a gradient of 50.

Normal anion gap metabolic acidosis with appropriate respiratory compensation.

b)

Mechanism is bicarbonate loss from GI tract as urinary anion gap is negative.

Discussion

Let us dissect these results systematically.

  1. The A-a gradient is 51.4:
    PAO2 = (0.3 × 713) - (30 × 1.25) = 176.4;
    thus A-a = (176.4 - 125) = 51.4
  2. There is acidaemia.
  3. The PCO2 is a compensatory response.
  4. The SBE is negative, suggesting that there is a metabolic acidosis
  5. The respiratory compensation is adequate:
    The expected CO2 is (15 × 1.5) + 8 = 30.5mmHg
  6. The anion gap is normal:
    (138) - (118 + 15) = 5, or 8.4 when calculated with potassium
  7. The urinary anion gap is (40+10) - 80 = -30

A low urinary anion gap suggests that there is no RTA, and that GI losses are responsible for the NAGMA.

But of course one could come to this conclusion by looking at the urine pH (which is near-maximally acidic, suggesting that appropriate renal compensation is taking place).

Thus, the mechanism must be gastrointestinal. Or, somebody has infused this 32-year-old male with an absurd excess of normal saline.

References

References

Batlle, Daniel C., et al. "The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis." New England Journal of Medicine 318.10 (1988): 594-599.