More metabolic acidosis than you can shake a stick at...

The set up

28 year old under going treatment for metastatic testicular cancer presents with a history of recurrent kidney stones.

pH 7.13

pCO₂ 22

pO₂ 96

Na 138

Cl 114

BUN 14

K 3.2

HCO3 8

Cr 1.0

glucose 96

urine lytes:

Na 56
Cl 78

K 12

Measured osmolality 292

Step one

Determine the primary acid-base disorder. The pH, bicarbonate and pCO2 are all moving in the same direction (down in this case). When all the Henderson-Hasselbalch variables are moving in the same direction (up or down) the primary disorder is metabolic. The pH is decreased so this is a metabolic acidosis. 

Step two

Is the compensation appropriate, or do we have a primary respiratory disorder as well as a metabolic acidosis? 
We use Winter's Formula to get the predicted pCO2 based on the bicarbonate.
1.5 x bicarbonate + 8 = 
1.5 x 8 +8 = 20

His actual pCO2 is 22 which is close enough, so a pure metabolic acidosis with appropriate respiratory compensation.

Step three

If there is a metabolic acidosis is there an anion gap?
138 - (114 +8) = 16
Yes, this is an anion gap metabolic acidosis.

Step four

If there is an anion gap, is there an osmolar gap? I usually don't bother to look for an posmolar gap uness the patient is particularly toxic with a large anion gap, neither of which describe Lance, but since the information includes the measured osmolality we should check this. You know, Chekhov's gun and all.
2 x Na + Glucose / 18 + BUN / 2.8 + Ethanol / 4.6 = calculated osmolality
2 x 138 + 96 / 18 + 14 / 2.8 + 0 / 4.6 = 286
Osmolar gap = measured osm – calculated osm

Osmolar gap = 292 – 286 = 6

This is a normal osmolar gap. Poor foreshadowing by the question writer.

Step five

If there is an anion gap, what was the bicarbonate before the anion gap? To calculate the bicarbonate before, take the anion gap, subtract 12 and add that to the current bicarbonate:

Bicarbonate before the anion gap = Bicarbonate + (Anion gap -12)

Bicarbonate before the anion gap = 8 + (16 –12)
Bicarbonate before the anion gap = 12

So the bicarbonate before the anion gap was 12 indicating a large non-anion gap metabolic acidosis and a relatively mild anion gap metabolic acidosis.

Step six

If there is an NAGMA, what is the urinary anion gap? What does it mean? The patient has a NAGMA as discovered in step 4. The differential of NAGMA is:

1. chloride intoxication
2. GI losses
3. RTA

The patient doesn't seem to be suffering from chlorine gas intoxication or have an isotonic saline drip running so number one is not likely.

The low potassium could indicate GI losses as well as type 1 or 2 RTA. The urine anion gap in the face of severe metabolic acidosis will help here. In GI losses and chloride intoxication the urine amnion gap will be negative, in RTA it should be positive.

Urine anion gap = (Na + K) – Cl

Urine anion gap = (56 + 32) – 78

Urine anion gap = + 10

The positive anion gap indicates a lack of NH₄⁺ in the urine. In diarrhea, the kidney will up ammonium excretion to get rid of the acid load. The increase cation load in the urine will be balanced by an increased in chloride in the urine. The increase Cl– will make the urine anion gap negative (in reality it is an unmeasured cation, or a positive cation gap, but by convention we use an anion gap). The positive urinary anion gap is the face of a severe acid load indicates a renal tubular acidosis.

Put it all together

This patient has a well compensated metabolic acidosis. The metabolic acidosis is partly anion gap and non-anion gap. The non-anion gap is a distal RTA. The AGMA may be a lactici acidosis from the neoplasm as these are not uncommon in metastatic neoplastic disease.

The chief complaint of kidney stones points to type 1 RTA. Patients with testicular cancer receive platinum-containing chemotherapy. Platinum can cause proximal or distal RTA. However, proximal RTA is not associated with kidney stones. So I suspect this  is classic distal RTA due to platinum.