Creatinine clearance calculators

[Christian Lancaster]

I’d like to share with you this interesting finding. It’s known that the calculation of CrCl using Cockroft Gault equation has some limitations:-

1- Weight (46.2 kg to 320 kg)
2- Age (older than 80 years)
3- Serum creatinine (0.5 mg/dL to 9.2 mg/dL)

Values out of the above ranges may result in over- or underestimation of real kidney function.

UpToDate calculator considers Actual Body Weight (ABW) while Micromedix calculator considers weight (actual body weight, Ideal body weight or Adjusted body weight) according to Body Mass Index (BMI) range rather than the actual weight. To understand, kindly check the attached table and pictures for a real case patient with weight of (44.8 Kg), age (101 years) and height of (160 cm). I changed the weights to (70 Kg) and (90 Kg) using the two calculators to check the difference in the resulting CrCl calculation. The age in this example causes an underestimation of the kidney function, however; it’ll show the difference in calculation. In such cases, using 24 Hr urinary creatinine gives more accurate calculation.

https://mega.nz/file/78QHCSZA#LtoGt8tLduilmrzuY1SinyGQlGIE3hCBHALL699xAbg

To conclude, knowing which weight to use is important for validity of the result.

[Rani Sudarshana]

Thank you for your effort in comparing all these calculators.
I agree with you about the importance of determining which body weight you have to use.
However,in you example you used an extremely old man which result in the following:
*It is well-known that geriatrics have a very low muscle mass even if they are obese which ideally means you have to use their ideal body weights or their actual body weigh (if they are underweight ) in the cleatinine calculations and NOT to use their actual (if they are not obese)or adjusted to prevent any over estimation of their renal function.

*Knowing that they have low muscle mass is very important specially for female geriatrics for this culculation.
* Knowing if the geriatric patient is diabetic will cause you to expect a further over estimation in your calculations for example if you calculate it to be 50 ml/hr and he is diabetic and your are dosing an antibiotic like vancomycin or an aminoglycoside …will prevent you from large increases in the doses…be cautious in your increment

*Because almost always with diabetes there is an underlined kidney disease that will not be detected by creatinine calculations but it has to be measured by 24hr collection. This is last statement is from a webinar of a nephrologist who spoke about the impact of DM on kidney function…. and also from my experience in dosing antibiotics in diabetic patients… I always remove 10 ml/hr from what I got from the calculation in diabetic patients if I got 50 ml/hr ,I do my dose calculations as if it is 40 ml/hr and if works fine with me and I hit the right dose from the first time.

* Another thing a lot of hospital lab softwares show the GFR and a lot of doctors count on it but actually in my experience it overestimate the renal function in geriatrics.
*Geriatrics means elderly above 70 but you can consider geriatrics anyone above 65 years old …I am saying so because I consider all the
co-morbidities that always available at this age like DM, Heart Failure, Hepatic Failure, CKD..which all impact the actual renal function. All these cause the patients to die around the age of 65.

[Christian Lancaster]

I’d like to thank Rani for the detailed and comprehensive reply which I’ll make sure to benefit from in my practice.

I’d like to clarify that the concern with UpToDate calculator is also valid for younger patients as it considers the patient weight rather than ideal or adjusted body weight when necessary in contrast to Micromedix calculator which adjusts weight as necessary.

Regarding the inquiry about how to assess renal function in AKI patients, the following are my thoughts:-

1- AKI is usually precipitated by causes that affect other factors related to production and excretion of creatinine which makes it difficult to use the commonly used equations that necessitate a stable creatinine level.

2- As per our practice, I recommend to combine two main parameters to judge the kidney function in patients with AKI:-

a) Urine output:- As a measure of the functionality of the kidney. Patient with anuria should be assumed to have a CrCl of less than 10 ml/min regardless of the equation result
b) Original MDRD equation:- As it contains parameters like (Albumin) and (BUN) which are usually altered by the conditions that can lead to AKI.

3- Consider the degree of AKI and treatment required, i.e Spontaneously resolving VS CRRT-managed; in adjusting the dose as per specific drug monograph.

To conclude, combining equations and clinical status of the patient is essential for drug adjustment in patients with AKI taking into consideration the management of precipitating factors for developing AKI.

[Author]

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