NephrologyTier 2Topic

Laboratory tests - renal function

The tests

Serum creatinine, estimated GFR (eGFR), and urea are the standard tests of renal function, each with distinct strengths and pitfalls
eGFR (CKD-EPI equation) is preferred over creatinine alone, since creatinine is affected by muscle mass, age, sex, and diet independent of true GFR
eGFR-based CKD staging is now standard practice; current equations have moved away from race-based coefficients

Creatinine lags true GFR change - a rise takes time to manifest after an acute insult, and a "normal" creatinine can mask significant nephron loss in someone with high baseline muscle mass
Urea rises disproportionately to creatinine with dehydration, GI bleeding (protein load), catabolic states, and steroid use - a raised urea:creatinine ratio suggests a pre-renal component
Cystatin C - an alternative marker less affected by muscle mass, increasingly used to refine eGFR estimation (combined creatinine-cystatin C equations) in specific situations (e.g. extremes of muscle mass, borderline staging decisions)

CKD is defined by abnormalities of kidney structure or function present for >3 months (eGFR <60, or markers of damage such as albuminuria) - a single abnormal result is not sufficient for diagnosis
AKI is defined by acute change (KDIGO criteria - creatinine rise, or oliguria) over hours-days, distinct from CKD

eGFR trend (not a single value) drives CKD staging, drug dosing decisions, and timing of RRT-related referrals (vascular access, transplant workup)
Report caveats: eGFR is less accurate at extremes of body habitus and muscle mass, and in acute kidney injury (equations assume steady-state)

Serial trend over months, not a single reading, is what defines CKD and its rate of progression

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