Case 19: Diagnostic Testing

Case 19 Index

A full set of labs are sent. The nurse is able to quickly run the arterial blood gas you have obtained while the other labs are in process:

pH 7.24 / pCO2 22 / pO2 92 / HCO3 15

 

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

Would you describe this patient’s arterial blood pH as acidemic or alkalemic?
Alkalemic
 The normal range for arterial blood pH is 7.35 – 7.45. A pH greater than 7.45 would be alkalemic. Try again!
Acidemic
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?
Respiratory
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!
Metabolic
That’s correct! The bicarbonate level is lower than normal (24 mEq/L), identifying a metabolic acidosis.
Is there appropriate compensation?
Yes
This patient does not seem to have an appropriate compensation based on Winter’s Formula. Try again!
No
Correct! The expected pCO2 based on Winter’s Formula would be 29-33 mmHg. There must be a secondary process going on here…

Your basic labs are back!

Here are some initial results:
Basic labs
Urinalysis

Basic Lab Ref

Based on the data you have acquired, which best describes this patient’s acid/base disturbance?
Anion gap metabolic acidosis, non-anion gap metabolic acidosis, and respiratory alkalosis
Not quite, try again! Hint: is there a delta/delta, or delta gap, for this patient?
Anion gap metabolic acidosis and respiratory alkalosis
That’s correct! This patient has a high anion gap (21) without a delta gap, pointing against a secondary metabolic process. He also has a pCO2 lower than would be expected based on Winter’s Formula, consistent with a concomitant respiratory alkalosis.
Anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis
Not quite, try again! Hint: is there a delta/delta, or delta gap, for this patient?
Anion gap metabolic acidosis and respiratory acidosis
Not quite, try again! Hint: what would be the expected pCO2 for a patient with a respiratory acidosis?
Non-anion gap metabolic acidosis and respiratory acidosis
Not quite, try again! Hint: the anion gap can be calculated using the following equation: AG = Na – Cl – HCO3
Non-anion gap metabolic acidosis and respiratory alkalosis
Not quite, try again! Hint: the anion gap can be calculated using the following equation: AG = Na – Cl – HCO3

 

Based on this patient’s acid/base status, pick 3 diagnoses at the top of your differential!
Salicylate toxicity
Diabetic ketoacidosis
Acetaminophen toxicity
Alcoholic ketoacidosis
Pulmonary embolism
Methanol ingestion
Renal failure
Lactic acidosis
Hypoventilation
Atorvastatin toxicity
Insulin toxicity

You transfer the patient to the Medical ICU for further management. The nurse asks if you would like any additional tests before the patient moves to the ICU.

Based on this patient’s acid/base disturbance, what additional tests would you like to send? Choose the highest yield!
Lactic acid
1.0 mmol/L. This is an important test to send in any patient with a metabolic acidosis.
Ethanol level
This patient’s ethanol level is <0.01 g/dL.
Creatine Kinase
25 U/L. We might expect this to be elevated in someone with rhabdomyolysis, but this patient’s urinalysis and basic labs do not seem consistent. While it’s not a bad test to send, there are definitely a few higher yield tests!
Acetaminophen level
This test will take some time to come back.
Serum osmolarity
It is always important to assess for an osmolar gap in patients with suspected ingestion. This patient’s measured serum osmolarity is 290 mOsm/kg. Is there an osmolar gap?
Salicylate level
This test will take some time to come back.
D-lactic acid
This lab is a send-out and will take 2-3 days to come back. While D-lactic acidosis cannot be completely ruled out, there are definitely a few higher yield tests!

 

Based on these results, what is your leading diagnosis? Lock one in before you move on!
Salicylate toxicity
Diabetic ketoacidosis
Acetaminophen toxicity
Alcoholic ketoacidosis
Pulmonary embolism
Methanol ingestion
Renal failure
Lactic acidosis
D-lactic acidosis
Hypoventilation
Atorvastatin toxicity
Insulin toxicity

Click here to confirm your diagnosis!

Case 19 Index
Case 19 Introduction
Case 19 Physical Exam
NephSim