The 2021 CKD-EPI Creatinine equation is a useful tool for estimating the kidney's filtering ability, known as the estimated glomerular filtration rate (eGFR). This measure is central to understanding kidney health and staging chronic kidney disease (CKD). Three inputs are needed: the person's age, gender, and serum creatinine level.

Here's how it works:

Inputs include the person's age, gender, and serum creatinine level. Age is provided as a whole number between 18 and 90 years. Gender options are "Male" or "Female." Serum creatinine levels can be entered in mg/dL (ranging from 0.1 to 20) or in µmol/L (ranging from 8.84 to 1768.4), with an internal conversion done to ensure units are in mg/dL for consistency.

The computation begins with a gender-dependent factor (A), which is 0.7 for females and 0.9 for males. Next, depending on whether the creatinine level is below or above this threshold, a second factor (B) is determined: for creatinine levels below or equal to the threshold, B is -0.241 for females and -0.302 for males; for levels above, B is -1.2 regardless of gender.

Using these factors, the eGFR is calculated with the formula: 
eGFR = 142 * (creatinine / A)^B * (0.9938^age)

Additionally, for females, the eGFR is multiplied by 1.012 for final adjustment. 

Once the eGFR is determined, it is rounded to a whole number and used to classify the CKD stage, reflecting the degree of kidney function:

- Stage 1 (Normal or High) is an eGFR of 90 or above
- Stage 2 (Mildly Decreased) spans 60 to under 90
- Stage 3a (Moderately Decreased) encompasses 45 to under 60
- Stage 3b (Moderately Decreased) covers 30 to under 45
- Stage 4 (Severely Decreased) ranges from 15 to under 30
- Stage 5 (Kidney Failure) is under 15

These stages provide critical insights into kidney health, potentially guiding clinical decisions and management.

The 2021 CKD-EPI creatinine equation estimates Glomerular Filtration Rate (eGFR) using serum creatinine, age, and sex. This updated version removed race as a variable to provide more equitable care across populations. It's currently recommended by KDIGO guidelines for routine clinical practice.

CKD-EPI Creatinine Equation for eGFR (2021)