Should the estimated glomerular filtration rate (eGFR) be adjusted for body surface area in a patient who is 193 cm tall and weighs 100 kg to avoid underestimating the true glomerular filtration rate?

Yes, you are correct—failing to adjust eGFR for body surface area in a patient of 193 cm and 100 kg will likely underestimate the true GFR.

For drug dosing and clinical decisions requiring absolute kidney clearance (in mL/min), you should "de-index" the standardized eGFR by multiplying it by the patient's actual BSA/1.73 m² to avoid underdosing medications in larger individuals.

Why This Matters

Standard eGFR equations (MDRD, CKD-EPI) report values normalized to 1.73 m² BSA (mL/min/1.73 m²), which is appropriate for:

• Diagnosing and staging CKD 1
• Comparing kidney function across populations 2

However, this indexed value systematically underestimates absolute GFR in patients with BSA >1.73 m² 3, 4.

The Clinical Impact in Your Patient

For a patient who is 193 cm tall and weighs 100 kg:

• Calculated BSA (using DuBois formula): approximately 2.3 m² 1
• If the indexed eGFR is 90 mL/min/1.73 m², the actual absolute GFR is: 90 × (2.3/1.73) = ~120 mL/min

This represents a 33% underestimation of true kidney clearance when using the indexed value alone.

Evidence Supporting De-indexing

The Canadian Society of Nephrology explicitly addresses this 3:

• Using indexed eGFR for drug dosing leads to underdosing in larger people and overdosing in smaller people
• They recommend "back calculation to an absolute clearance using an anthropometric estimate of the patient's body surface area in people who are clearly larger or smaller than the average 1.73-m² person"

Recent research confirms this 5, 6:

• In patients with BSA >2.0 m², indexed eGFR underestimated measured GFR by up to 23.59% 6
• De-indexing improved accuracy (P30) from 42% to 74% in obese patients 5
• For carboplatin dosing in cancer patients with BMI >30, concordance improved from 26% to 65% when using de-indexed values 5

Current Guideline Recommendations

The 2025 KDOQI commentary 4 and 2024 KDIGO guidelines 7 now explicitly state:

Practice Point 4.2.4: "In people with extremes of body weight, eGFR non-indexed for body surface area (BSA) may be indicated, especially for medications with a narrow therapeutic range or requiring a minimum concentration to be effective" 4.

The NKF-ASN Task Force 8 and recent consensus statements 9 emphasize:

• Indexed eGFR can differ significantly from non-indexed measured GFR in patients with high BSA
• Average indexed eGFR was 19.7-20.9 mL/min lower than non-indexed measured GFR in the highest BSA group 8
• This is particularly problematic given rising obesity prevalence, especially in certain populations 8

Practical Implementation

For drug dosing in your 193 cm, 100 kg patient:

1. Calculate actual BSA using DuBois formula: BSA = 0.007184 × (weight in kg)^0.425 × (height in cm)^0.725 1

2. De-index the eGFR: Absolute GFR (mL/min) = eGFR (mL/min/1.73 m²) × (patient's BSA/1.73) 3, 4

3. Use this absolute value for:

   • Chemotherapy dosing (especially carboplatin, which uses GFR-based formulas) 1
   • Medications with narrow therapeutic windows 4
   • Any renally cleared drug where underdosing could compromise efficacy 3

Important Caveats

• For CKD staging and diagnosis, continue using indexed eGFR 1, 2
• In patients with extreme obesity (BMI >30), even de-indexed equations may remain inaccurate (P30: 62-100%) 10—consider measured GFR with inert tracers or therapeutic drug monitoring in these cases
• The Cockcroft-Gault equation, while historically used for drug dosing, is now considered obsolete and should be replaced with de-indexed CKD-EPI equations 9, 10

Bottom line: Your clinical intuition is correct—using the standard indexed eGFR in this tall, heavy patient will underestimate his true kidney clearance and potentially lead to medication underdosing. De-index the eGFR for drug-related decisions.