FAS (Full Age Spectrum)

Creatinine equation for children, adults and older adults

Biomarkers
Creatinine
Validated for
2+ years
Inputs
Age, Sex, Creatinine
Reports
mL/min/1.73 m²
Published
2016

The eGFR guide explains which equation the calculator suggests and indexed and absolute GFR.

Formula

For ≤40y:eGFR=107.3Scr/Q; For >40y:eGFR=107.3Scr/Q×0.988age−40\text{For } \leq 40y: \text{eGFR} = \frac{107.3}{S_{cr}/Q}; \text{ For } > 40y: \text{eGFR} = \frac{107.3}{S_{cr}/Q} \times 0.988^{\text{age}-40}

As text: For ≤40y: eGFR = 107.3 / (SCr / Q); For >40y: eGFR = 107.3 / (SCr / Q) × 0.988^(Age−40)

Constants

SymbolMeaningValue
107.3Base coefficient
QQMedian serum creatinine of healthy persons of the same age/sex (Pottel 2016, Table 1)
Female (≥20 y): 0.70mg/dL
Male (≥20 y): 0.90mg/dL
2–19 y: Age-dependent (see Pottel 2016, Table 1)mg/dL
0.988Age decay factor (age > 40 only)per year
ScrS_{cr}Serum creatinine (input)mg/dL
AgePatient age (input)years

How it was developed

FAS was not derived by statistical modelling: its coefficients were set mathematically so that a paediatric equation joins the BIS1 equation for older adults without a break. It was validated in 14 data sets from Europe and the USA (6,870 people), with GFR measured as urinary inulin or iothalamate clearance, or plasma iohexol clearance.

Clinical notes

  • Validated with IDMS-equivalent serum creatinine
  • The 0.988 age factor equals a decline of about 1 mL/min/1.73 m² per year
  • The paper also gives height-based Q values for children; this calculator uses age

Limitations

  • Validated in Caucasians only; Q values for other ethnicities not yet established
  • Overestimates GFR at very low creatinine, such as severe muscle wasting
  • Underestimates GFR in adults aged 70 or older with measured GFR ≥60

References

  1. Pottel H, et al. An estimated glomerular filtration rate equation for the full age spectrum. Nephrol Dial Transplant. 2016;31(5):798-806. doi:10.1093/ndt/gfv454

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