Return to Blog Homepage

Mystery Solved: The Most Commonly “Pimped” Lab Questions Med Students Should Know

This article is meant to help guide you on how to interpret and manipulate labs correctly before being told to do so.

Sometimes it may be enough to just report the labs as they are. Other times, it is crucial to take things one step further. Read this article and report your labs with these additions, and you’ll be sure to impress your team before they have to ask you to make these changes!

You have a patient with a low calcium…

The causes of hypocalcemia are numerous (i.e. medications, malignancy or vitamin D) and this is a topic for a different blog. Before we get into the cause, how about answering these two commonly asked questions:

“Is the calcium actually low?” “Did you correct for albumin?”

In the body, calcium is found in two major forms: ionized and bound to protein (albumin).  The standard lab test measures the total amount of calcium in the body.  However, it is the ionized calcium that impacts calcium-induced symptoms (i.e. if the ionized calcium is low, then this is true hypocalcemia). If the albumin is low, then less calcium is bound to albumin.  This means that the measured total calcium from the lab will be lower than expected, however the ionized calcium may in fact be completely normal.  To determine if ionized calcium is truly low, we need to do some math.  For every 1 g/dL of albumin that is lower than normal, you must add 0.8 mg/dL to the total calcium level to correct for this “falsely” low calcium the lab reported.  Let’s try it.  If the albumin is 1 g/dL less than normal and your calcium is 8.0, then you need to add 0.8 mg/dL to your calcium for a corrected calcium of 8.8 mg/dL.  There are multiple online calculators available if you are in a rush, but you should be able to make sense of the physiology and math on your own. Always remember to correct calcium for albumin before you present your patients in the morning!

You have a patient with a pleural effusion that gets tapped…

Perhaps you were even able to watch the thoracentesis (I recommend watching as many procedures as possible as long as your team is okay with it–you’ll learn a ton!). The question you’ll get asked when you present the results is:

“Is the effusion transudative or exudative?”

Light’s criteria helps to determine if the effusion is transudative or exudative.  This is important because this categorization provides for the first branch point in the long differential for pleural effusions. Transudative effusions are commonly CHF, cirrhosis, or nephrotic syndrome.  Meanwhile, exudative effusions tend to be infectious or malignant.  Therefore, knowing this categorization of pleural effusions can help you plan out your management steps and then make your presented assessment and plans more clear! An effusion is exudative if at least one of the following is true:

  1. Effusion LDH > â…” upper limit of serum LDH
  2. Effusion LDH/serum LDH >0.6
  3. Effusion protein/serum protein >0.5

You have a patient with a low WBC count…  

Many patients in the hospital can have a low WBC counts, and again the differential for leukopenia is long.  However, regardless of the differential, the question you’ll get asked for your leukopenic patient is this:

What is the patient’s ANC?

ANC stands for absolute neutrophil count.  An ANC helps to determine if a patient is truly neutropenic or not.  The calculation is the total WBC x %(bands + PMNs).  An ANC<1500 defines neutropenia.  This is important because neutropenia and fever in a patient who is immunosuppressed, for instance on chemotherapy, is a medical emergency.  If you are calling a consult to a hematologist/oncologist, be sure to mention the ANC.  Also, just to review, leukopenia is the term for a reduced total WBC count, and it should not be confused with neutropenia.

You have a patient with an acute kidney injury…

AKI is a very common hospital diagnosis. There are many criteria for the definition of AKI, but if you see a marked increase in a patient’s creatinine, think AKI and look at the different criteria. The question you’ll get asked when you present the results is:

“What is the patient’s FeNa?”  

FeNa is the fractional excretion of sodium.  It is calculated in percent format and the formula is (urinary sodium x serum creatinine)/(serum sodium x urinary creatinine).  We will talk about renal issues in more depth in another blog, but the basics are as follows.  A FeNa <1 is more of a prerenal causes such as dehydration.  A FeNa between 1 and 2 can be prerenal, intrinsic kidney injury, or acute tubular necrosis.  Finally, a FeNa >2 usually indicates acute tubular necrosis.

Here is a wrinkle: your patient is on a diuretic. Wouldn’t that effect the sodium excretion? Yes, it absolutely would. So here is the next question you will be asked for those patients with AKI who are on diuretics:

Did you calculate an FeUrea?”

Patients on diuretics have impaired sodium handling due to mechanism of the drug trying to rid the body of sodium and thus water.  Therefore, FeNa is not useful in these patients on these medications.  You must instead calculate a FeUrea, as Urea is not effected by diuretics.  It is calculated in percent format the same way FeNa is calculated with urea replacing the sodium; the formula is (urinary urea x serum creatinine)/(serum urea x urinary creatinine).  A FeUrea <35% indicates a pre-renal cause while a FeUrea of 50-65% indicates acute tubular necrosis.

You have a patient that comes into the ED with presumed sepsis…

Many patients in the hospital will come in with various kinds of infections.  However, you will categorize their illness based on some criteria.  Therefore, the question you’ll get asked when you present the results is:

Does the patient meet SIRS Criteria?”

SIRS stands for systemic inflammatory response syndrome. It is composed of 4 criteria and a patient meets SIRS criteria if they have two or more criteria.

  1. Temp >38C (100.4F) or <36C (96.8F)
  2. Heart rate >90
  3. Respiratory rate >20 or PaCO2 <32 mmHg
  4. WBC >12,000/mm3, WBC <4,000/mm3, or >10% bands

Naming the SIRS criteria was perhaps my most pimped question of third year.  I have them written down on a notecard in my white coat pocket! Now if a patient meets SIRS criteria but also has a source of infection (say evidence of pneumonia on a chest XRAY), then the patient is said to have sepsis.  Severe sepsis is when the patient also has organ dysfunction/hypoperfusion and hypotension leading to lactic acidosis, SBP<90 mmHg or a SBP drop >40 mmHg of normal. Finally, septic shock criteria is met when a patient has severe sepsis with hypotension that does not improve with fluids.

 

Hopefully this article will help you succeed on the wards.  Before you present, check your labs to make sure you do not have to make any of the manipulation/corrections above.