The 2021 CKD-EPI Creatinine-Cystatin C calculator is designed to estimate an individual's kidney function through the calculation of the estimated glomerular filtration rate (eGFR) and to assign a chronic kidney disease (CKD) stage based on key health parameters. By inputting a person's age, gender, serum creatinine, and cystatin C levels, the calculator determines their eGFR and corresponding CKD stage. Here is how it works:

- **Serum Creatinine**: This is initially adjusted to mg/dL if provided in µmol/L, using a straightforward conversion: creatinine (mg/dL) = creatinine (µmol/L) / 88.4.
  
- **Gender-Specific Parameters**: The calculator uses different parameters for males and females due to physiological differences:
  - For females:
    - A = 0.7
    - B depends on whether creatinine is ≤ 0.7 (value -0.219) or > 0.7 (value -0.544)
    - C = 0.8
    - D depends on whether cystatin C is ≤ 0.8 (value -0.323) or > 0.8 (value -0.778)
  - For males:
    - A = 0.9
    - B depends on whether creatinine is ≤ 0.9 (value -0.144) or > 0.9 (value -0.544)
    - C = 0.8
    - D depends on whether cystatin C is ≤ 0.8 (value -0.323) or > 0.8 (value -0.778)

- **Calculating eGFR**: With these values, the eGFR is calculated using the formula:
  \( eGFR = 135 \times (creatinine / A)^B \times (cystatin C / C)^D \times 0.996^{\text{age}} \)
  If the individual is female, this result is further adjusted by multiplying the calculated eGFR by 0.963 to account for differences in muscle mass and body composition.

- **CKD Stage Interpretation**:
  - eGFR ≥ 90 indicates Stage 1 (Normal or High renal function)
  - eGFR 60-89 indicates Stage 2 (Mildly Decreased function)
  - eGFR 45-59 indicates Stage 3a (Moderately Decreased function)
  - eGFR 30-44 indicates Stage 3b (Moderately Decreased function)
  - eGFR 15-29 indicates Stage 4 (Severely Decreased function)
  - eGFR < 15 indicates Stage 5 (Kidney Failure)

By evaluating the inputs of age, gender, creatinine, and cystatin C levels, this calculator yields a rounded eGFR and a concise assessment of kidney health through the assigned CKD stage. This interpretation aids in diagnosing and managing potential kidney health issues, based on recognized medical guidelines.

The 2021 CKD-EPI Creatinine-Cystatin C equation combines both serum creatinine and cystatin C measurements to estimate GFR. This combination provides improved accuracy over single-marker equations, especially in patients where creatinine-based estimates alone may be less reliable.