UWorld Test-Taking Strategy: Acid-Base

  • /Reviewed by: Amy Rontal, MD
  • USMLEWorldTestTakingStrategy A healthy 45 year old male travels to Switzerland for a mountain climbing trip. During the trip, he stays in a high-altitude camp in the mountains at over 14,000 feet above sea level. After 6 days, he has blood drawn as part of a research study. Which of the following arterial blood gas values would you expect to see with this sample:

    pH  PaO2 (mm Hg) PaCO2 (mmHg) Plasma HCO3
    A: 7.25 98 21 11
    B: 7.32 60 55 31
    C: 7.38 70 40 22
    D: 7.49 60 20 15
    E: 7.50 96 30 17
    F: 7.6 72 50 32

    When I was studying for Step 1, I would initially panic when I saw questions like this, thinking, “Not only are you asking me to pick between six answers, you’re asking me to work through several variables to arrive at the correct choice!”

    Now, as a tutor, I see a similar form of anxiety in my students when they encounter questions like these or the dreaded up/down arrows often seen in Endocrinology. However, what I am here to show you is that with some very simple test-taking strategies, you can make questions like these a strength when taking Step 1. Let’s break it down into a series of steps:

    Step 1:

    I always recommend that students start by reading the question at the end of the vignette first, and then very quickly looking at the answer choices. This can help put the question in context from the start and allow you to more easily decipher clues along the way. Here, we quickly see that this is an acid base question. Now, going through the vignette, it’s clear that this is an acid base question about the effects of high altitude.

    Step 2:

    Make your own answer. In questions like this, I encourage my students to ask, “What would I expect to see?” In this question, that requires understanding the physiological effects of high altitude. Briefly, remember that at high altitude, due to the declining PaO2, our body develops a compensatory tachypnea. As a result, we are blowing off more CO2, which will subsequently lead to an increase in pH. This is respiratory alkalosis. As with all acid base questions, the next issue becomes assessing for compensation. At this point, 6 days in to his trip, we would expect the patient in this vignette to have metabolic compensation by decreasing his bicarbonate. Now, returning to the four variables in this vignette, we can say that the correct answer will have a pH > 7.4, a low PaO2, a low PaCO2, and a low HCO3.

    Step III:

    Now it’s time for process of elimination. As we have diagnosed a respiratory alkalosis, we can instantly eliminate answer choices A, B and C as these all have acidotic pH’s. Moving on to PaO2, we can now reasonably eliminate answer choice E as we would not expect a PaO2 of 96 at a 14,000 foot elevation. In the span of a few seconds, we have taken the amount of possible answers from six to two! Now, let’s make the final choice.

    Step IV:

    We are now left with answer choices D and F. So, which is it? Again, as we have been dealing with a respiratory alkalosis, we know that the correct answer will have a low PaCO2. Thus, we can eliminate F and are left with D. However, before selecting this answer, we must confirm that the final variable, HCO3, fits with our choice. Here, given that the patient has been at altitude for 6 days, we would expect that metabolic compensation has occurred, thus, we would expect to see a low HCO3. A HCO3 of 15 as seen in answer choice D allows us to definitively know that we have made the correct choice.

    As you can see, creating our own answer before picking one of the many choices kept us from panicking at so many options and allowed us to rely on process of elimination to confirm a correct response.

    Now, let’s take a look at another strategy for acid base with a new question:

    A 25 year-old female is brought to the emergency department with worsening weakness and altered mental status. She also notes new onset polyuria and polydipsia, weight loss, and urine with a strong, fruity odor. Her blood pressure is 95/70 mm Hg and her pulse is 125/min and regular. Her laboratory results are as follows:

    Serum Chemistry

    Sodium 134 mEq/L
    Potassium 3.5 mEq/L
    Chloride 100 mEq/L
    Bicarbonate 12 mEq/L
    Creatinine 1.2 mg/dL
    Glucose 498 mg/dL
    pH 7.27
    PaCO2 40mm Hg

    Based on the lab findings, which of the following is most likely occurring in this patient?

    A: Increased urinary bicarbonate loss
    B: Respiratory failure
    C: Type IV Renal Tubular Acidosis
    D: Renal tubular necrosis
    E: Low plasma renin activity
    F: Adrenal insufficiency

    So how do we tackle this one?

    Step 1:

    Again, let’s start by looking at the question. While not as instructive as the previous one, we know that we are going to be looking for a pathophysiologic mechanism to explain the diagnosis in the vignette. As we now read through the question, we are given several clues to suggest DKA: polyuria, polydipsia, fruity smelling urine. Given the hemodynamic instability, altered mental status, and acidotic pH, we know that we are likely dealing with DKA. But now, let’s make sure all of the labs fit.

    Step 2:

    One thing my students constantly hear from me is to make sure that all of the symptoms, lab findings, and imaging present in the question fits the answer choice you are picking. If one variable doesn’t add up, then you are likely wrong, or there is something else going on. Here, we definitely see symptoms that could be explained by DKA. But, what about the labs? A potassium of 3.5 fits as we know that patients with DKA have depletion of intracellular and total body stores of potassium, but often normal, or low normal potassium levels on a serum chemistry. Also, a bicarbonate of 12 mEq/L makes sense given that DKA is a metabolic acidosis. Additionally, glucose of 498 mg/dL and an anion gap of 22 further supports the diagnosis of DKA. But, what about the PaCO2?

    Step 3:

    Where many students get tripped up, is seeing a PaCO2 of 40 mm Hg and overlooking it because it is in the normal range of lab values or casually glossing over it as they have already made the diagnosis of DKA. However, I encourage my students to not view this as normal. Again, we return to the issue of compensation. In a metabolic acidosis like DKA, a normal response would be a compensatory tachypnea in an attempt to lower our PaCO2 and to try to raise our pH to some degree. As such, a PaCO2 of 40 mm Hg should certainly stand out and alert you to the presence of a mixed disturbance which helps us arrive at the correct answer here.

    These two questions highlight that having good test-taking skills and developing a strategy for attacking questions is an integral part of achieving a high score on your USMLE Step I. Continue checking back for more test-taking strategies and find new ways to improve your score!