5 Misconceptions About eGFR That New Pharmacists Should Master

[This article is written for the following people.]

This may be sudden, but when you studied eGFR for the national exam,
didn't you think like this??

......I get it. I really do.
But once you get out into the field, it feels a bit different.

Me: "The eGFR is 40, so I should reduce the dose, right?"

Senior: "Well, it's not just that..."

Have you ever had an experience like this?
I was told this by my senior when I was a rookie.
I had confirmed it with full confidence, like 'Ta-da!', but I was shot down.

Certainly, eGFR is a convenient indicator.
If you have serum creatinine, age, and gender, you can get it immediately.
It's also easy to use because it's automatically displayed in the electronic medical record.

That is precisely why,it is easy to 'think you understand it'.

When evaluating renal function,
there is one major premise I want you to grasp first.

The 'e' in eGFR stands forestimated,in other words,'estimation'
.It is not a measured value.

It is an estimated value calculated from a formula,
and for about 75% of cases, the error from the measured GFR stays within ±30%.
For the remaining 25%, there is a possibility of a larger deviation.

"Wait, it deviates that much?"

Yes. It does deviate!
It is, after all, just anindicator.

First, with this premise in mind,
let's organize the '5 misconceptions' that are easy to stumble overtogether.

Misconception 1: It is okay to design dosage based on eGFR (mL/min/1.73㎡) as is

The eGFR displayed in the electronic medical record.
Are you checking that number, including the units?

--Yes, this is a value corrected for a body surface area of 1.73㎡.
This unit is used for CKD diagnosis and severity classification.

However, do not use it for drug dosage design!

What you should use for dosage design is
the unadjusted eGFR (mL/min)
with the body surface area correction removed, or Ccr (creatinine clearance).

Why is that?

Think about it.
A large-framed man who is 180cm tall and weighs 90kg, and
a small-framed elderly woman who is 150cm tall and weighs 45kg.
Both have a body surface area of 1.73㎡.

If you use it while it is still corrected for body surface area,
you end up ignoring the influence of body size.

Suppose the eGFR (mL/min/1.73㎡) is the same at "50";
is the amount of drug actually flowing through the kidneys the same?

If you decide the dosage using the corrected value,
there is a possibility of underdosing for a large-framed person, and conversely,
overdosing for a small-framed person.

The point to check in clinical practice is simple.

First, check whether "unadjusted eGFR" is displayed in your hospital's electronic medical record.
If it is not displayed, you need to calculate it yourself.

When I was a newcomer, I didn't understand this.
And there were many times later on when I struggled because of it.
That's why I really think you should understand at least this!

Instead of thinking "It's OK because the eGFR is shown,"
the habit of checking "which eGFR it is" is something you should develop while you are a newcomer, and it will make your work much easier later on.

Misconception 2: Dosage adjustment is complete if you only look at eGFR

I checked the eGFR.
I also thoroughly checked whether it was unadjusted.

"Alright, now I can decide the dosage!"

......Wait a minute!

eGFR is only one aspect of renal function.
Especially for elderly patients with low muscle mass and edema, who are common in convalescent rehabilitation hospitals, interpreting eGFR becomes much more difficult.

For example, a case like this.

If muscle mass is low, serum creatinine will appear low.
In other words, eGFR can sometimes look better than it actually is.

As a result, administering a standard dose leads to the drug being too effective, causing side effects.

There is also the opposite pattern.

The treatment for the infection does not work sufficiently,
leading to what is known as asubtherapeuticstate.

eGFR is merely the starting point.

In addition to that, you want to check urine volume, weight fluctuations, edema, electrolytes (especially potassium), BUN,
and
the renal excretion rate of the drug being administered and the width of its therapeutic range.

I want you to change how you frame the question.

To take an extreme example,
you don't really worry about the dosage of probiotics even for someone with poor kidney function, right?
But don't you worry about the dosage of anticoagulants?

The therapeutic range is narrow, and both overdose and underdose are dangerous.
Be cautious with such drugs.

It takes a little extra effort, but this effort will protect the patient.

Misconception 3: Anyone can interpret eGFR the same way just by plugging it into a formula

Did you know that the eGFR calculation formula is adjusted for Japanese people?

If you input serum creatinine, age, and gender,
anyone will get the same number when they calculate it.

…That is thebiggest pitfall.

Even if the number produced by the formula is the same,
what that number means differs depending on the patient.

Even with the formula for Japanese people, there are cases where accuracy decreases.
Extreme obesity, emaciation, pregnancy, children
—such patients fall outside the assumptions of the formula.

What you want to be especially conscious of in convalescent rehabilitation is
elderly people with extremely low muscle mass.

Since serum creatinine is a metabolic byproduct of muscle,
the production amount itself is low in patients with sarcopenia.

If you don't understand this, you might think,
Creatinine is low → eGFR appears high → 'Kidney function is preserved.'

But in reality, kidney function might not be that good.

That is why I want you to have a perspective that makes you pause here.

Just by asking that question, what you see will change.

When what you see changes,
・Check Cystatin C when you are concerned
・Consult with a nephrologist or a senior pharmacist
—this leads to that 'next move'.

I believe this is truly, truly important.

Misconception 4: The eGFR value perfectly represents 'kidney function at that moment'

Open the electronic medical record and check today's eGFR.

Wait a minute!
There is one thing I want you to check.

Did you compare it with the eGFR from last week or last month?

When I was a rookie, I didn't have this habit.
I always looked at eGFR as a spot check and said it was 'good' or 'bad'.
Because of that, I once made an absurd proposal to the attending physician and got scolded.

eGFR is information from 'that point in time' when the blood was drawn.
But kidney function needs to be viewed as a video, not a still image.

For example, in acute kidney injury (AKI),
kidney function fluctuates significantly over a few days.

Even if today's eGFR is 40, if it was 55 a week ago,
it might bein the middle of a sharp decline.

After contrast media administration, dehydration, sepsis
—in a situation where 'it might get even worse from here on',
it is dangerous to decide the dosage based only on today's number.

You also cannot overlook the opposite trend.

65 a year ago, 55 six months ago, and 40 today
——a gradual decline over the yearsin a patient
is an independent risk factor in itself.

A convalescent rehabilitation hospital is an environment where you can be involved with the same patients for a long time.
This means blood test data will accumulate repeatedly during their hospital stay.

This is alsoa perfect opportunity to track trends.
Beyond just "today's eGFR,"
I really want you to develop the habit of looking at "this person's eGFR journey"
while you are here.

Misconception 5: If eGFR is within the reference range, you don't need to worry much about kidney risk

That judgment is half correct.
But the other half is dangerous.

Even with an eGFR of 60 or higher,it does not mean the risk is zero.
For example, if urinary albumin is increasing
or the rate of eGFR decline is rapid,
the kidneys may already be sending out an SOS even if the numbers are in the "normal range."

The principle of CKD evaluation is
【a combination of eGFR andurinary albumin (proteinuria)】

A judgment cannot be completed with just one number.

Another point to keep in mind is the relationship between CKD and cardiovascular disease.
It is known that as eGFR decreases, the risk of cardiovascular events and all-cause mortality increases.
It is not just a kidney issue; it also needs to be viewed as systemic risk management.

Many patients admitted to convalescent rehabilitation hospitals
have underlying diseases such as diabetes, hypertension, and heart disease.

If there is a background of "10 years of diabetes, hypertension, and proteinuria" behind the number "eGFR 65, normal range," the story changes completely.

Don't just look at the numbers,look at the patient behind the numbers.
eGFR is a tool for that.
We don't want to be used by our tools, do we?

An "eGFR Checklist" to avoid confusion in clinical practice

Let's summarize what you want to check before designing a dosage.

It's not difficult.

Stop for a moment and think, "Can I apply this number to this patient as it is?", that's all.

What you want to check is,

• physical build (is there extreme obesity or emaciation?)

• Age and the presence or absence of sarcopenia

• The type of eGFR displayed (with or without body surface area correction)

• Recent trends (comparison with past values)

• Urinalysis (protein/albumin)

• The renal excretion rate of the drug to be administered and the dosage adjustment section of the package insert

—Just these six points.

It does not mean you have to judge everything by yourself.

If you are unsure, consult a senior pharmacist or a certified nephrology pharmacist. Just deciding on that timing for yourself will make it much easier to act.
Just deciding on that timing for yourself will make it much easier to act.

Summary: How to deal with eGFR that you want to master while you are a newcomer

eGFR is a truly convenient indicator.

But if you read today's article and
felt a little bit of 'fear,' that is fine.

I want you to have an attitude of 'not over-relying on it just because it is convenient.'

Do not take the formula results at face value,
but judge them in conjunction with the patient's overall picture.

Look at the trends.
Look at the physique.
Look at the background diseases.

I believe that stacking these up one by one
is the job of a pharmacist who faces renal function.

Convalescent rehabilitation is a place where you can continue to be involved with the same patients for a long time.
Blood test data is updated every week, and you can feel the changes in eGFR firsthand.

It is also an environment where you can learn 'how to read eGFR' that is not taught in the national examination, little by little in the field.

If you become friends with eGFR while you are a newcomer,
it will definitely help you later.
Why not start with just one thing first?

▶If you want to know more about renal function evaluation, this article might also be helpful.

▶You want to calculate renal function easily, right? You can use the calculation page on the Japanese Society of Nephrology and Pharmacotherapy website for free. Please give it a try.