Case 25: Diagnostic Testing

Case 25 Index

While we’re waiting for some basic labs, let’s start with the arterial blood gas

pH 7.24 / pCO2 27 / pO2 90 / HCO3 12

Let’s follow the steps below to see what this patient’s acid-base disturbances are (think pLACO):

Would you describe this patient’s arterial blood pH as acidemic or alkalemic?
 The normal range for arterial blood pH is 7.35 – 7.45. A pH greater than 7.45 would be alkalemic. Try again!
That’s right! The normal range for arterial blood pH is 7.35 – 7.45. This pH less than 7.35 is acidemic.
What is the primary process that led to the pH change?
A respiratory acidosis would be the consequence of a high pCO2. In this case the pCO2 is lower than normal (40 mmHg), meaning it is not the primary process driving the acidemia. Try again!
That’s correct! The bicarbonate level is lower than normal (24 mEq/L), identifying a metabolic acidosis.
Is there an elevated anion gap?
Oops, try that one again!
Correct! This patient’s normal anion gap can be calculated using the albumin (3.9 g/dL) and multiplying it by 2.5, which gives us 10. The anion gap, or measured anions, can be calculated using: Na – Cl – HCO3 = 136 – 115 – 12 = 9. As this is close to 10, there is no elevated anion gap.  Thus, this patient has a non-anion gap metabolic acidosis. Remember to always calculate the anion gap, even if the bicarbonate is elevated or normal. 
Is there appropriate compensation?
We can look for respiratory compensation for a metabolic acidosis using Winter’s formula: expected pCO2 (within 2 mm Hg) = 1.5*HCO3 + 8 = 1.5 (12) + 8 = 26 mm Hg – this is within 2 of the patient’s pCO2 of 27 mm Hg.  We do have adequate respiratory compensation. 
Using Winter’s formula, we can calculate the expected pCO2 if adequate compensation is present. 
Based on the acid/base disturbances, pick 3 diagnoses at the top of your differential!
Salicylate toxicity
Diabetic ketoacidosis
Acetaminophen toxicity
Alcoholic ketoacidosis
Surreptitious laxative use 
Type 1 renal tubular acidosis (RTA)
Type 2 RTA
Type 4 RTA
Surreptitious diuretic use
Lactic ketoacidosis
Ethylene glycol poisoning
Vomiting due to chemotherapy 

Let’s move on and get some more data

Case 25 Index
Case 25 Introduction
Case 25 Physical Exam