Lab case 442 interpretation


From the archive


Question 1 

PH = 7.236, that is moderate acidaemia

HCO3 = 12.5 mmol/L. So we have metabolic acidosis.

Next we need to calculate the compensation and the the anion gap.

We calculate the compensation using Winter’s formula [expected PCO2 = 1.5 x HCO3 +8 (+/- 2)]. Accordingly, expected PCO2 should be between 25 and 29. PCO2 here is 30. That is very close. The patient might have a very minor small respiratory component.

AG = Na – (Cl + HCO3) = 3.5, SO we have NAGMA.

Other abnormal findings

K = 2.7 mmol/L. (2.7 is in the range of moderate hypokalaemia), However, we should correct potassium level according to PH. ( Potassium shifts to intravascular space so increasing the serum K). K level usually increases by 0.6 for every 0.1 PH below 7.4.

Accordingly, Corrected K = 2.7 – (0.164 x 0.6)/0.1 = 1.7 and that puts the level in the range of severe hypokalaemia.

Cl = 124 (Hyperchloraemia).

The final conclusion is Hyperchloraemic  NAGMA with severe hypokalaemia.

 Question 2 answer

For NAGMA, considering the USED CARP mnemonic,
U—Ureteroenterostomy. (Excluded by history taking)
S—Small bowel fistula. (Excluded by history taking)
E—Extra chloride. (No fluid loss, normal Na, Normal BSL, Normal Creatinine-unlikely to be the cause).
D—Diarrhea. (Excluded by history taking)
C—Carbonic anhydrase inhibitors. (Excluded by history taking)
A—Adrenal insufficiency/ Addison’s disease (Normal Na, low K), unlikely.
R—Renal tubular acidosis. Possible (Usually present as hyperchloraemic NAGMA)
P—Pancreatic fistula. (Excluded by history taking)

The only possible explanation is RTA.

Next step is to find out what type of RTA

There are 4 types of Renal Tubular Acidosis (RTA).

  1. Type 1 (Distal), due to reduced H+ secretion at the distal tubules. Usually causes Low K, HCO3 < 15 and urinary PH > 5.5.
  2. Type 2(Proximal), due to impaired HCO3 absorption in proximal tubules. Usually causes low K, HCO3 > 15 and urinary PH < 5.5.
  3. Type3 (proximal and distal), extremely rare.
  4. Type4 due to impaired Cations exchange at the distal tubules. Usually causes high K, HCO3 > 15 and urinary PH < 5.5.

Accordingly, we have type 1 RTA. This is usually associated with renal calculi.


 Question 3 answer

Untreated type 1 RTA leads to growth retardation in children and progressive kidney and bone disease in adults. Restoring normal growth and preventing kidney stones are the major goals of therapy. Acidosis is usually corrected with sodium bicarbonate or sodium citrate, this will lead to correction of low potassium, salt depletion, and calcium leakage into urine.

Alkali therapy usually leads to decrease the risk of  kidney stones and stabilizes kidney function so kidney failure does not progress. Infants may need potassium supplements, but older children and adults usually don’t  because alkali therapy prevents the kidney from excreting potassium into the urine.