An 84-year-old male with a recent diagnosis of myeloma and osteoarthritis is admitted to ICU following a three-day history of constipation followed by diarrhoea.


Patient Value

Adult Normal Range





7.35 – 7.45


98 mmHg (13 kPa)


10.0 mmHg (1.3 kPa)*

35.0 – 45.0 (4.6 – 6.0)




4.0 mmol/L*

22.0 – 26.0

Base Excess

-22.0 mmol/L*

-2.0 – +2.0


1.4 mmol/L

0.5 – 1.6


133 mmol/L*

135 – 145


5.7 mmol/L*

3.5 – 5.0


113 mmol/L*

95 – 105


4.4 mmol/L

3.5 – 6.0

a)    Describe the acid base abnormality.    (10% marks)

b)    Give a physiological rationale for the acid base abnormalities in this patient.
(20% marks)

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College answer

Describe the acid base abnormality. (10% marks)

High anion gap metabolic acidosis, respiratory alkalosis, delta ratio of 0.2 suggesting associated normal anion gap metabolic acidosis.

Give a physiological rationale for the acid base abnormalities in this patient (20% marks)

HAGMA without elevated lactate in this scenario may be renal failure (multiple possible causes) or starvation ketosis

Coexisting NAGMA from diarrhoea

Respiratory alkalosis from hyperventilation secondary to pain/distress

Only one rationale per abnormality required.


Let us dissect these data;

  • The A-a gradient is close to zero. 
  • There is acidaemia
  • There is a metabolic acidosis (SBE = -22)
  • The CO2 is lower than it should be: the expected value is 18 mmHg (using the SBE method) or 14 mmH (using Winter's formula).
  • The anion gap is (133  + 5.7 ) - (113  + 4.0) = 21.7, or 16 when calculated sans potassium.
  • The delta ratio is (21.7-12)/(24-4.0) = 0.49, or 0.2 if older values are used (where the standard anion gap value is 12, and the gap is calculated without potassium). 

This is weird. Irrespective of which formula you use, the delta ratio range associated with a mixed disturbance is probably 0.4-0.8 (according to Brandis), as the older  0.8-1.2  range had poorer sensitivity (according to Rastegar, 2007). Either way, delta ratio of 0.2 is not expected to be associated with a mixed disorder. Taken literally, this value suggests that the rise in the anion gap accounts for only 20% of the drop in the bicarbonate. Using updated values and incorporating the nontrivial potassium (5.7mmol/L) one arrives at a delta ratio of 0.5 or so, which is more consistent with a mixed disturbance. 

Physiological rationale for the acid base abnormalities in this patient:

There are multiple possible causes for a mixed distrubance. Individually, the causes of metabolic acidosis are: 

High anion gap

Normal anion gap



This old man with constipation and diarrhoea could potentially have any number of these causes, considering that we have zero history. Renal failure is a plausible cause of this acid-base disturbance because the acidosis is usually mixed in that setting, and the history suggests some fluid depletion.  



Rastegar, Asghar. "Use of the ΔAG/ΔHCO3− Ratio in the Diagnosis of Mixed Acid-Base Disorders." Journal of the American Society of Nephrology 18.9 (2007): 2429-2431.

Dinubile, MarkJ. "The increment in the anion gap: overextension of a concept?." The Lancet 332.8617 (1988): 951-953.