As an RN, you’ll find that you need these four basic calculations almost every single shift you work. In this post we’ll cover what they are and why they matter.

Anion Gap

The anion gap will usually come into play when you are taking care of a patient in diabetic ketoacidosis. However, to be totally accurate, it is actually used to alert us that the patient is in ANY kind of metabolic acidosis and can even help us differentiate what caused it.

Recall that some electrolytes are positively charged, while others are negatively charged. The main cations (positively-charged electrolytes) we are concerned about are Sodium molecules, while the main anions (negatively-charged) are Chloride and Bicarbonate. The anion gap formula is:

anion gap

A normal anion gap is 8-16 mEq/L…another higher than 16 indicates that an anion gap is present and your patient is in metabolic acidosis. Generally the cause will be diabetic ketoacidosis, sepsis/lactic acidosis or renal failure.

When the gap is < 8 mEq/L, it may be because your Na is low (hyponatremia), albumin is low, or it could be multiple myeloma.

For example, your patient has the following values on their chem panel:

  • Na = 140
  • Cl = 116
  • HCO3 = 6

But wait! HCO3 doesn’t show up on a chem panel! Ringie dingie dingie!! We have a winner!!! Great catch! When you don’t have an ABG and you want to know what the HCO3 is for purposes of assessing acid/base balance, you use the serum CO2 value. Pretty cool, huh?

Let’s say you’re taking care of a patient in diabetic ketoacidosis…you will be calculating the anion gap every four hours when you send off for your chemistry panel. When the anion gap is normal we say that the “gap is closed” and the treatment for DKA changes course at that point.

So, in the above example, your patient’s anion gap is going to be: 18. Is their gap open or closed? It is open…good job!

For more about DKA, check this out or listen to this Podcast!

P/F Ratio

The P/F ratio is going to come into play when you are worried about a shunt situation or if your patient is in ARDS. For this calculation you only need to know two numbers….your PaO2 (available from your ABG) and the FiO2 (how much oxygen the patient is on). To do this calculation, you need to convert your FiO2 to a decimal…so if they’re on 40% FiO2, then it will be 0.40.

If your patient’s PaO2 is 75 and their FiO2 is 80%, what’s the P/F ratio? it’s 93.75….that’s really bad, by the way! How about if the PaO2 is 90 and the FiO2 is 21%? It’s 428…MUCH better!

Basically, a P/F ratio < 300 indicates Acute Lung Injury (ALI), while a P/F ratio < 200 is bad news and suggestive of ARDS. Anytime it gets in the 280 or below range, you’re definitely going to be thinking that there’s some kind of shunt going on. For more info about shunts, check out this post on V/Q mismatching…for more info about the P/F ratio, look here!

Corrected Calcium

Keeping electrolytes in balance is a key role of the RN, especially in the critical care setting when ALL SORTS of spooky things can happen. If you don’t have the benefit of obtaining an ABG (and, thus, an ionized calcium) then you’ll want to be sure to “correct” the serum calcium, especially in the presence of low albumin levels. Why is this?

Remember that most calcium (about 80%) is bound up with albumin where it doesn’t do much except sit around and look cool. So if you’ve got a patient whose chem panel says they’ve got low calcium AND low albumin…is this a true representation of their physiologically-active calcium? This “unbound” or “ionized” calcium is calculated by correcting for the low albumin level, like so:

Though the accuracy of correcting your calcium level is being brought into question, if you don’t have an ABG then this is essentially the best you’re going to get. Note that some equations replace the 4.0 with a 4.5…you can always do the calculation both ways and get a range for what your “corrected calcium” actually is.

Note that it’s not just cases of hypocalcemia w/hypoalbuminemia that you’ll want to adjust…do the same in cases of hypercalcemia w/hypoalbuminemia as well…just to be sure you’re getting a value that closely resembles what’s actually going on with your patient.

BUN:Cr Ratio

When you look at your BUN and creatinine together you want to determine the BUN:Cr ratio in order to identify the cause of your patient’s renal failure. When your ratio is > 20 this is typically due to pre-renal issues; and when it’s < 10, it’s typically because of problems within the kidney itself. Generally speaking, a level 13ish-20 is considered normal. Here’s the super simple calculation:

BUN Cr ratio

For example, let’s say your patient’s BUN is 26 and the creatinine is 4.0…the answer is 6.5, which indicates the problem is within the kidney…maybe they have nephrotic syndrome or a bad kidney infection, or just straight up renal failure.

But what if your BUN is 36 and your creatinine is 1.6? The ratio fraction is 22.5, indicating the problem is pre-renal…maybe dehydration or hypovolemia. Interested in learning more about taking care of patients in renal failure? Look no further…I’ve got you covered!

In addition to these four very useful calculations, you must also be confident in your ability to do dosage calculations. If you’re fuzzy on that or want to learn a FOOLPROOF method that works with every type of calculation EVERY time, check out this post here!

What other calculations do you see in the clinical setting? How’s clinical going? If you’re an RN, how’s the job going? Let us know in the comments below!


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nursing calculations you'll use every day