Kidney Function Interpretation in Athletes: Why Creatinine Misreads the Muscular Population, the Cystatin C and eGFR Adjustments, and the Differential Against Rhabdomyolysis, NSAID Use, and Supplement Stacks

A 31-year-old male powerlifter walks into intake six weeks after a routine annual physical. He weighs 95 kg at roughly 12% body fat. His primary-care physician ran a basic metabolic panel as part of the visit. Serum creatinine came back at 1.4 mg/dL. The reference range on the lab report is 0.7-1.3 mg/dL. The PCP flagged the value as elevated, calculated an eGFR-CKD-EPI of 62 mL/min/1.73m², and labeled the result 'stage 2 chronic kidney disease.' He was told to repeat the panel in three months, to stop creatine supplementation, to stop his pre-workout because it contains caffeine, to avoid NSAIDs, to reduce his protein intake from his current 2.0 g/kg per day to 'no more than 0.8 g/kg per day,' and to consider a nephrology referral if the creatinine did not normalize. He has been off creatine for six weeks. His protein intake is now hovering at 1.0 g/kg per day. He has lost 1.8 kg of muscle on the scale and his squat is down 18 kg. He is in the office because his coach told him the prescription does not pass the smell test and to find a sports RD who reads labs through the right frame.

The kidney-function workup that produced his diagnosis was the wrong instrument for his presentation. A serum creatinine of 1.4 mg/dL in a sedentary 31-year-old male is the upper end of the reference range and, paired with an eGFR around 62, is consistent with early-stage chronic kidney disease that warrants the workup the PCP initiated. The same creatinine value in a 95-kg powerlifter at 12% body fat is consistent with a 50-pound muscle-mass premium above the reference population that produced the equation — and an eGFR estimate that is systematically biased downward by an amount the equation cannot correct for, because the equation was never built to. The athlete's measured GFR, run formally in nephrology six weeks later via iohexol clearance, came back at 118 mL/min/1.73m². His kidneys were normal. The 'CKD stage 2' label was an artifact of reading a muscle-mass-confounded biomarker through an equation calibrated on a population without his body composition. The intervention he was prescribed — protein restriction, creatine discontinuation, eight weeks of muscle loss — was iatrogenic harm on a clinically false-positive diagnosis.

Most sports-RD intakes do not run a structured kidney-function workup. The BMP arrives from primary care, the creatinine flag is the headline, the eGFR estimate gets read at face value, and the athlete is told either that his kidneys are fine (when they are not, in the rhabdomyolysis or NSAID-injury differential) or that his kidneys are not fine (when they are, in the muscle-mass-artifact differential). The interpretive frame that distinguishes the body-composition artifact from the real renal case, and that runs the supplement-and-medication overlay before reaching a workup conclusion, is rarely on the intake worksheet. The athlete who needed a cystatin C confirmation ends up on protein restriction. The athlete who had early rhabdomyolysis from a heavy training week gets reassured by a normal-looking creatinine that was actually elevated against his own baseline. Both errors are common. Both are preventable with the structured workup.

This post is the kidney-function workup I run when a BMP shows up in the sports-nutrition intake. The six-dimension interpretive frame, the confounder list that explains most of the false positives, the cystatin C and measured GFR alternatives that catch the real cases, the rhabdomyolysis and NSAID differential, the supplement-stack overlay, the medical-coordination triggers, the common counseling mistakes, and the SOAP pattern that documents the case defensibly.

Why the standard creatinine read misses athlete physiology

Three structural reasons.

Serum creatinine is a function of muscle mass and renal clearance, and the standard read treats it as a function of clearance alone. Creatinine is the breakdown product of phosphocreatine in skeletal muscle. The rate at which it appears in the serum is proportional to total skeletal-muscle mass; the rate at which it disappears is proportional to glomerular filtration. The serum concentration at steady state is the ratio of the two. The reference range (typically 0.7-1.3 mg/dL for adult males, 0.6-1.1 for adult females) was calibrated on a sedentary, mixed-body-composition population whose muscle mass clusters in a narrow band relative to renal clearance. A 95-kg athlete at 12% body fat carries roughly 80 kg of lean mass and produces creatinine at a rate well above the reference population — his serum creatinine at the same clearance is mechanically higher than the reference population's. The reference range read at face value flags him as 'elevated' for a reason that has nothing to do with his kidneys.

The eGFR equations propagate the creatinine artifact without correcting for it. The CKD-EPI and MDRD equations estimate glomerular filtration rate from serum creatinine, age, sex, and (in older versions) race. None of the variables in the equation captures muscle mass directly. The equation assumes a population-average muscle-mass-per-kg-body-weight relationship that high-muscle-mass athletes systematically violate. The result is an eGFR estimate that is biased downward in athletes — frequently by 20-40 mL/min/1.73m² — and that bias does not represent a real reduction in glomerular filtration. The eGFR flag of 'stage 2 CKD' on the lab report is a function of the equation's structural assumption, not of the athlete's actual renal function.

The same biomarker is the rate-limiting signal for the cases that the standard workup misses entirely. Acute kidney injury from rhabdomyolysis, NSAID-induced nephritis, contrast-induced nephropathy after imaging, and pre-renal injury from severe dehydration are all real entities in the athletic population, and all of them present with creatinine values that may still fall inside the reference range if the athlete's baseline was low. The athlete whose baseline creatinine is 1.0 mg/dL and whose post-marathon creatinine is 1.3 mg/dL has a 30% rise in serum creatinine that is consistent with acute kidney injury by KDIGO criteria — but the 1.3 falls inside the 'normal' reference range and the standard read passes it as unremarkable. The case that requires urgent intervention gets reassured. The case that requires reassurance gets restricted.

The six-dimension kidney-function workup

Dimension 1: Read serum creatinine against the athlete's body composition, not against the population reference range. A serum creatinine of 1.3-1.5 mg/dL in a high-muscle-mass male athlete (BMI 25-30 with confirmed body fat under 18%, or BMI 30+ with confirmed body fat under 22%) is consistent with the muscle-mass premium and should not be read as a renal flag in the absence of a clinical correlate. The same creatinine in a sedentary 70-kg male is a renal case until proven otherwise. The first interpretive question is whether the lean-mass denominator of the creatinine production-to-clearance ratio is sitting outside the reference population — and if it is, the standard read is not the right instrument.

Dimension 2: Read against the athlete's own baseline, not against the population reference range. The most clinically useful kidney-function comparison is the athlete's current value against her own previous value at the same training state. A 50% rise in serum creatinine from baseline meets KDIGO criteria for acute kidney injury regardless of whether the absolute value falls inside the reference range. A 20% rise warrants attention and a recheck. An athlete with a baseline of 0.8 mg/dL and a current value of 1.2 mg/dL has a 50% rise that the reference-range read will pass as 'normal' and that the longitudinal read flags correctly as an acute event requiring workup. The sports-RD intake that captures historical lab values and reads the current draw against the athlete's own trajectory catches the cases the cross-sectional read misses.

Dimension 3: Add cystatin C when muscle mass confounds the creatinine read. Cystatin C is a low-molecular-weight protein produced at a constant rate by all nucleated cells, freely filtered at the glomerulus, and not reabsorbed. Its serum concentration depends on glomerular filtration and is largely independent of muscle mass, age, sex, and dietary protein. The cystatin-C-based eGFR (the 2021 CKD-EPI cystatin equation or the combined creatinine-cystatin equation) is the better-calibrated estimate of glomerular filtration in athletes whose creatinine read is suspect. For the powerlifter case in the opening, a cystatin C draw would have produced an eGFR-cystatin in the 100+ range, immediately resolving the false-positive flag without the iohexol-clearance workup. Cystatin C is the single highest-yield add-on to a kidney workup in athletes and the standard PCP panel almost never includes it. The sports-RD case-management work is to request the addition through the athlete's primary care or to suggest it before any 'reduced function' conclusion gets locked in.

Dimension 4: Run the supplement, medication, and hydration overlay. Five common athletic-population exposures can raise serum creatinine for reasons that do not reflect glomerular injury and that the standard workup does not flag. Creatine monohydrate supplementation raises serum creatinine by approximately 0.1-0.3 mg/dL at standard loading and maintenance doses, by raising the substrate pool that produces creatinine without affecting glomerular function. NSAIDs (ibuprofen, naproxen, ketorolac) reduce afferent arteriolar perfusion via prostaglandin-mediated vasodilation suppression and can produce a real, reversible reduction in glomerular filtration that resolves on discontinuation. Severe dehydration produces pre-renal azotemia with rising creatinine and rising BUN-to-creatinine ratio above 20:1. Cooked-meat consumption immediately before the draw raises serum creatinine by absorbing pre-formed creatinine from the muscle of the meat. Acute heavy resistance training in the 24-48 hours before the draw raises serum creatinine via muscle breakdown that resolves with recovery. Document each of these before reaching any interpretive conclusion. The athlete who creatine-loaded six weeks ago and has been on 5 g/day maintenance is producing a serum creatinine that runs roughly 0.2 mg/dL higher than her off-creatine baseline, every time, by mechanism that has nothing to do with her kidneys.

Dimension 5: Run the rhabdomyolysis differential when the clinical context warrants. Acute heavy training, two-a-days, eccentric-dominant sessions, returning to training after a layoff, high-altitude or high-heat exposure, dehydration, and certain medications (statins, fluoroquinolones) all raise the rhabdomyolysis differential. The lab markers are creatine kinase (CK) — typically >5,000 IU/L for clinically significant rhabdo, with values into the 50,000-100,000+ range in severe cases — paired with myoglobinuria (positive heme on urine dipstick without RBCs on microscopy), elevated AST and LDH, hyperkalemia, hyperphosphatemia, hypocalcemia, and the acute creatinine rise that follows. The case that presents with dark-cola-colored urine, severe muscle pain disproportionate to recent training, and weakness needs the rhabdo workup ordered through urgent care or emergency medicine, not a recheck in three months. The sports-RD role is the differential trigger, not the workup itself, but the trigger has to be recognized at the intake or the case progresses to the late presentation where acute kidney injury is established.

Dimension 6: Read BUN, BUN-to-creatinine ratio, electrolytes, and urinalysis together. A BMP includes BUN, electrolytes, and bicarbonate alongside creatinine, and a basic urinalysis is the cheapest add-on to disambiguate the differential. BUN-to-creatinine ratio above 20:1 with normal electrolytes suggests pre-renal physiology (dehydration, GI bleed, high-protein intake with otherwise normal renal function). Ratio under 10:1 suggests intrinsic renal physiology or BUN-disproportionate processes (advanced liver disease, low protein intake). Urinalysis with proteinuria flags glomerular pathology; with hematuria flags glomerulonephritis or stone disease; with glucosuria at normal serum glucose flags proximal tubule dysfunction; with positive heme on dipstick without RBCs on microscopy flags myoglobinuria from rhabdo. The four-marker read (BUN, creatinine, electrolytes, urinalysis) is the minimum-viable kidney workup; reading creatinine alone leaves most of the differential unaddressed.

The four-quadrant clinical decision matrix

The six dimensions collapse into a four-quadrant matrix that drives the action plan.

Quadrant 1: Elevated creatinine, body-composition-consistent, no clinical correlate. Serum creatinine 1.3-1.5 mg/dL in a high-muscle-mass male athlete (or 1.0-1.2 in a high-muscle-mass female athlete), normal BUN-to-creatinine ratio, normal urinalysis, no symptoms, no rhabdo trigger, no NSAID exposure. The presumed muscle-mass-artifact case. The action is the cystatin C confirmation — request via primary care, read the cystatin-eGFR against the cystatin reference range, and document the resolution. No intervention indicated; do not restrict protein, do not discontinue creatine, do not start the CKD workup until cystatin C says the muscle-mass-artifact hypothesis is wrong.

Quadrant 2: Elevated creatinine, body-composition-consistent, with a confounder present. Same body composition, with active creatine loading, recent heavy training, NSAID use, or significant dehydration documented. The action is to remove the confounder and recheck — discontinue creatine for 4-6 weeks (with the athlete's awareness of the temporary detraining cost), allow 72 hours of recovery and rehydration before the recheck, then reassess. If the creatinine normalizes, the confounder explained the rise; if it does not, escalate to the cystatin C confirmation and the quadrant-1 workup.

Quadrant 3: Elevated creatinine, body-composition-inconsistent, or rising from athlete's baseline. Creatinine elevated against the athlete's previous draws by ≥30%, or absolute value outside the body-composition-consistent range, or paired with clinical symptoms (edema, fatigue disproportionate to training, hypertension, oliguria, dark urine). The real renal case until proven otherwise. Order or request through primary care the full workup — repeat BMP with cystatin C, urinalysis with microscopy and ACR (albumin-to-creatinine ratio), kidney ultrasound if indicated, nephrology referral. The sports-RD intervention runs in parallel with the medical workup and includes a temporary modest reduction in protein toward the upper-normal range (1.2-1.6 g/kg per day from 2.0+ g/kg) pending the workup outcome, NSAID discontinuation if applicable, optimized hydration, and creatine pause pending the workup.

Quadrant 4: Acute creatinine rise with rhabdomyolysis triggers or clinical signs. Recent heavy/eccentric training, dark urine, severe muscle pain, weakness, or any CK value above 5,000 IU/L if measured. The acute kidney injury case until ruled out. Refer to urgent care or emergency medicine immediately. The sports-RD role is the recognition and the referral, not the workup. Post-acute the case transitions to medical co-management with the sports-RD intervention focused on hydration, gradual return to training, and the recovery-nutrition plan calibrated to the medical recovery cadence.

When to refer to medical

Five signals warrant medical referral beyond the dietetic workup.

Creatinine ≥ 50% above the athlete's documented baseline. The KDIGO threshold for acute kidney injury. Refer regardless of absolute value or muscle-mass interpretation.

Creatinine paired with proteinuria on urinalysis. Glomerular pathology. Nephrology evaluation indicated regardless of muscle-mass interpretation.

Any of the rhabdomyolysis presentation triggers. Dark urine, severe muscle pain, weakness, recent extreme training, or any context where CK is likely above 5,000 IU/L. Emergency medicine or urgent care referral is the appropriate pathway.

Hypertension in a young athlete with elevated creatinine. The secondary-hypertension differential includes renal-parenchymal and renal-vascular causes. Primary care or nephrology evaluation indicated.

Family history of polycystic kidney disease, IgA nephropathy, Alport syndrome, or other hereditary kidney disease, paired with any creatinine elevation. Hereditary-disease screening warrants nephrology evaluation regardless of absolute values.

Common counseling mistakes

Reading the eGFR estimate at face value without questioning the equation's body-composition assumption. The CKD-EPI equation was not built to handle the high-muscle-mass athlete. The flag of 'reduced function' on the lab report does not survive the cystatin C confirmation in most athletic cases, and the protein-restriction prescription that follows from accepting the flag at face value produces iatrogenic muscle loss for a clinically false-positive diagnosis.

Discontinuing creatine reflexively for any creatinine elevation. Creatine raises serum creatinine by mechanism that has nothing to do with glomerular injury. The standard 0.1-0.3 mg/dL elevation on standard maintenance dosing is expected, reproducible, and reversible. The clinical question is whether the elevation exceeds what creatine supplementation explains, not whether creatine should be stopped on principle. The athlete in real renal injury needs to stop creatine; the athlete with a creatine-attributable elevation does not.

Counseling protein restriction below the athletic requirement for a presumed kidney case before the diagnosis is confirmed. The 0.8 g/kg per day population recommendation that often gets cited in CKD counseling is calibrated on a sedentary population with established kidney disease. Applying it to an athlete with a suspect creatinine elevation produces real harm — loss of lean mass, reduced training capacity, compromised recovery — for a diagnosis that frequently does not survive the cystatin C confirmation. The intervention sequence is the diagnostic confirmation first, the targeted intervention second.

Failing to capture the athlete's baseline creatinine for longitudinal comparison. The most clinically useful kidney-function read is the trajectory, not the cross-section. The intake that does not capture historical lab values and read the current draw against the athlete's own baseline cannot distinguish the acute event from the chronic muscle-mass artifact.

Skipping the supplement and medication overlay before reading the creatinine. Five common athletic exposures raise serum creatinine for non-renal reasons. The intake that does not run the overlay before interpretation is reading the lab through a confounded signal.

Counseling 'increase your hydration' as the catch-all kidney-protection strategy without context. Hydration matters for pre-renal physiology and for the prevention of stone disease, contrast nephropathy, and the dehydration-driven rhabdo case. It is not a meaningful intervention for the glomerular pathology case, and 'drink more water' as the default counseling can produce hyponatremia in the endurance athlete who follows it during a long event. The hydration counseling is calibrated to the clinical context, not blanket.

Failing to coordinate with the athlete's primary care or nephrology on the cases that warrant escalation. Kidney function is medical management. The sports-RD intervention runs in parallel with the medical workup, not in place of it. The chart trail has to show the coordination.

Where this lands in the SOAP

Subjective section format:

```

Kidney Function Workup (panel reviewed YYYY-MM-DD):

• Serum creatinine: [X mg/dL, lab reference range, draw date]
• BUN: [X mg/dL, lab reference range]
• BUN/Cr ratio: [X]
• eGFR-CKD-EPI (creatinine): [X mL/min/1.73m²]
• eGFR-CKD-EPI (cystatin C): [X mL/min/1.73m² or NOT ORDERED]
• Electrolytes: [Na, K, Cl, HCO3]
• Urinalysis: [protein, blood, glucose, dipstick + microscopy summary, or NOT ORDERED]
• ACR (albumin/creatinine ratio): [X mg/g, or NOT ORDERED]
• CK: [X IU/L if measured, or NOT INDICATED]

Athletic-physiology context:

• Body composition: [BMI, body fat %, method, date]
• Estimated lean mass: [X kg]
• Training state: [in-season / off-season / recent heavy training]
• Hydration status: [intake estimate, urine color, body-weight trend]
• Creatine supplementation: [yes/no, dose, duration]
• NSAID exposure: [agent, dose, frequency, indication]
• Recent meat-protein intake before draw: [timing, quantity]
• Pre-draw training in 24-48 hours: [session type, intensity]
• Symptoms: [edema / fatigue / muscle pain / dark urine / oliguria / hypertension / none]
• Baseline creatinine if available: [X mg/dL, prior draw date]
• % change from baseline: [X% rise/fall]

Quadrant: [1-4 from clinical matrix]

Confounder summary: [list of muscle-mass / creatine / NSAID / hydration / training / meat exposures]

Missing values: [cystatin C / ACR / urinalysis, plan to request via PCP]

Clinical impression: [statement integrating creatinine with body composition, baseline, confounders, and clinical context]

Action: [cystatin C confirmation / confounder removal and recheck / medical referral / urgent referral for rhabdo workup / no intervention indicated]

Follow-up: [recheck date, plan, escalation triggers]

```

Assessment integrates the kidney panel with the body-composition data, the supplement and medication overlay, the symptomatic and training picture, and the longitudinal trajectory. Plan documents the differential reasoning, the medical-coordination communications, and the recheck cadence. See [SOAP notes for sports dietitians](/blog/soap-notes-for-sports-dietitians) for the broader documentation framework.

The female-athlete kidney-function thread

Female athletes carry their own interpretive nuances. The reference range for serum creatinine is lower (typically 0.6-1.1 mg/dL) because population-average muscle mass is lower, and the same body-composition-artifact logic applies in reverse — a 70-kg female athlete at 16% body fat with documented high lean mass can carry a creatinine of 1.0-1.2 mg/dL that the standard read flags as elevated and the body-composition-adjusted read passes as expected. The cystatin C confirmation is the same disambiguation tool. The interpretive frame is identical; only the absolute thresholds shift downward.

Female endurance athletes additionally carry a non-trivial risk of exercise-associated hyponatremia in long events, with the kidney-function workup contributing to the differential when fluid-overload presentations emerge. The standard intake should capture the longest-event nutrition and hydration plan and read any kidney workup against the recent endurance-event exposure history.

The parallel iron and menstrual-cycle workups — the [iron-status workup](/blog/iron-status-workup-in-female-athletes) and the [menstrual-cycle and contraceptive-status workup](/blog/menstrual-cycle-charting-in-female-athlete-intake) — run alongside the kidney workup as part of the integrated female-athlete intake, and the cases where multiple systems are flagged warrant the integrated read rather than the single-panel pickup.

The masters-athlete kidney-function trajectory

Masters athletes (typically 50-plus) carry rising baseline prevalence of age-related GFR decline, frequently overlaid on the same body-composition picture as younger athletes. The masters athlete with a creatinine of 1.3 mg/dL is harder to disambiguate than the 25-year-old with the same value — both the body-composition premium and the age-related decline can be present, and the cystatin C confirmation matters more, not less. The longitudinal read against the athlete's own baseline is the most reliable instrument; the cross-sectional read against the population reference range routinely misclassifies both directions in this population.

Additional masters-athlete considerations: NSAID use is more common (chronic joint complaints, cumulative training history), the medication overlay is broader (statins, ACE inhibitors, ARBs, diuretics — all with kidney-relevant pharmacology), and the cardiometabolic context is more variable. The workup discipline matters more in this population, and the medical-coordination cadence is typically tighter.

Where platform tooling helps

The bottleneck in the kidney-function workup at scale is the multi-variable integration — the creatinine read against the athlete's body composition and her own baseline, the eGFR equation flagged for the body-composition assumption it makes, the cystatin C add-on requested through the medical team when the body-composition-artifact hypothesis warrants confirmation, the supplement and medication overlay captured at intake, the rhabdomyolysis differential checked against the recent training history, the urinalysis paired with the BMP for the four-marker workup, the longitudinal trajectory tracked across draws, and the medical-coordination communications logged across the care team. The intake that does all of this by hand drops the integration on busy weeks and the case surfaces months later either with the athlete on inappropriate protein restriction or with the real renal case progressed past the recoverable window.

The leverage is a kidney-function workup module that ingests the panel values, captures the body-composition data, runs the muscle-mass-artifact interpretation against the creatinine reference range, prompts for cystatin C when the artifact hypothesis is plausible, captures the supplement and medication overlay, flags the rhabdomyolysis triggers, pairs the BMP with the urinalysis, tracks the longitudinal trajectory across draws, pre-populates the SOAP documentation, and tracks the medical-coordination communications. The RD's job becomes the clinical judgment and the conversation, not the spreadsheet.

The chart trail is also defensible — every interpretation tied to the body-composition and confounder data that justified it, every medical referral documented with the workup data that drove it, every intervention paired with the differential reasoning that produced it.

The bottom line

Kidney-function panels in athletes carry a different interpretive load than in the sedentary population. Serum creatinine reflects muscle mass at least as much as it reflects renal clearance, and the eGFR equations propagate the muscle-mass confound without correcting for it. The standard read flags the high-muscle-mass athlete as 'reduced function' for a body-composition reason that has nothing to do with her kidneys, and the protein-restriction prescription that follows produces real iatrogenic harm on a clinically false-positive diagnosis. At the same time, the same biomarker reads as 'normal' in the acute kidney injury case where the absolute value rose 30-50% from the athlete's own baseline but stayed inside the population reference range, and the case that needed urgent intervention gets reassured.

The workup that catches both errors reads creatinine against the athlete's body composition and her own baseline, requests the cystatin C confirmation when the body-composition-artifact hypothesis is plausible, runs the supplement and medication overlay before reaching a conclusion, holds the rhabdomyolysis differential against the recent training history, pairs the BMP with a urinalysis for the four-marker workup, and coordinates with the medical team on the cases that warrant escalation.

The 31-year-old powerlifter with a creatinine of 1.4 mg/dL and a 95-kg body mass at 12% body fat is not a CKD case until cystatin C says he is. The 22-year-old marathoner whose creatinine moved from 0.8 to 1.2 mg/dL post-event is an acute kidney injury case even though both values fall inside the reference range. The intake that runs the structured workup distinguishes the two. The intake that reads the eGFR flag at face value collapses them into the same wrong diagnosis.

[Calsanova's Dietitian plan](/signup?role=dietitian) ships a kidney-function workup module with panel-value ingestion, body-composition-adjusted creatinine interpretation, cystatin C escalation logic, supplement-and-medication overlay capture, longitudinal trajectory tracking, rhabdomyolysis trigger flagging, and pre-populated SOAP documentation with medical-coordination communications. Start your 30-day free trial and turn the kidney panel from a single-number flag into a clinical instrument that catches the muscle-mass artifacts and the real acute injury cases that the standard read collapses together.