Blood Work Guide
KIDNEY FUNCTION & ELECTROLYTES
Your kidneys filter waste, balance fluids, and quietly keep dozens of systems running. Here is what BUN, creatinine, eGFR, and your electrolyte panel actually tell you — and when an “abnormal” result is nothing to worry about.
Medically reviewed by Missy Zammichieli, DNP, APRN, FNP-BC ยท Updated March 2, 2026
THE BOTTOM LINE
Your kidneys filter waste, balance fluids, and quietly keep dozens of systems running. The kidney panel checks how well they are doing that job. Most “abnormal” results on this panel come down to one of three things:
- • Dehydration — the single most common cause of flagged results
- • High protein intake — elevates BUN through normal metabolism
- • Extra muscle mass — raises creatinine without affecting actual kidney function
If a value is flagged, your provider will look at the full picture before drawing conclusions. The single best thing you can do for accurate results is drink plenty of water before your blood draw.
INTRODUCTION
Your kidneys do more than filter waste. They balance fluids, help control blood pressure, signal your body to make red blood cells, and activate vitamin D. They filter roughly 150 quarts of blood every day, and most of that work happens without you noticing.
Kidney function can decline significantly before symptoms appear — most people feel nothing until they have lost 70–80% of function. That is why this panel exists: to catch problems early. This article explains each marker. It is educational, not diagnostic — your results should always be interpreted by a clinician who knows your full history.
YOUR BIOMARKERS EXPLAINED
BUN (Blood Urea Nitrogen)
What it measures: When your body breaks down protein, a waste product called urea ends up in your blood. BUN measures how much is there.
Why it matters: Your kidneys clear urea, so BUN rises when filtration slows down. But it is also pushed up by high-protein diets, dehydration, and GI bleeding — so it is not purely a kidney marker.
Reference range: 7–20 mg/dL.
Note for active individuals: A high-protein diet can push BUN to the upper end of the range or slightly above. That reflects protein metabolism, not kidney damage.
Creatinine
What it measures: A waste product from the normal breakdown of creatine in your muscles. Production is fairly steady based on how much muscle you carry, and your kidneys filter nearly all of it out.
Why it matters: Because production is steady, creatinine is a more reliable kidney marker than BUN. When kidney function drops, creatinine builds up.
Reference range: 0.74–1.35 mg/dL (men); 0.59–1.04 mg/dL (women).
What raises it: Decreased kidney function, higher muscle mass, dehydration, intense exercise, high meat intake, creatine supplementation, certain medications.
Why this matters if you train: A muscular 200-pound athlete who trains regularly will naturally produce more creatinine than a sedentary 140-pound person. A creatinine of 1.3 mg/dL in a competitive CrossFit athlete may be perfectly normal, while the same value in a sedentary elderly person could signal a problem. This is exactly why eGFR was developed — to put creatinine in context.
eGFR (Estimated Glomerular Filtration Rate)
What it measures: A calculation (not a direct measurement) that uses your creatinine, age, and sex to estimate how well your kidneys filter blood.
Why it matters: The best single estimate of kidney function from routine blood work, and the basis for CKD staging.
| Stage | eGFR (mL/min/1.73 m²) | What It Means |
|---|---|---|
| G1 | 90 or above | Normal or high |
| G2 | 60–89 | Mildly decreased |
| G3a | 45–59 | Mild to moderate decrease |
| G3b | 30–44 | Moderate to severe decrease |
| G4 | 15–29 | Severely decreased |
| G5 | Below 15 | Kidney failure |
Important: A single eGFR below 60 does not mean you have CKD. The guidelines require that decreased eGFR persist for more than 3 months before that diagnosis is made.
Limitations for athletes: Because eGFR uses creatinine, it can underestimate kidney function in muscular people. A strength athlete with a creatinine of 1.4 mg/dL might get an eGFR of 65 (G2), yet their actual filtration is completely normal. Your provider can order a cystatin C-based eGFR — a version not affected by muscle mass — to confirm.
Sodium
What it measures: The main mineral in the fluid outside your cells. Primary driver of blood volume and fluid distribution.
Why it matters: When sodium is abnormal, the issue is usually water balance — too much or too little fluid — rather than a true sodium problem.
Reference range: 136–145 mEq/L.
High sodium: Dehydration, inadequate water intake, excessive sweating. Low sodium: Overhydration (a real risk for endurance athletes who overdrink during events), heart failure, kidney disease, certain medications.
Potassium
What it measures: Potassium lives mostly inside your cells. Only about 2% is in the bloodstream, so small shifts can swing your lab value significantly.
Why it matters: Essential for heart rhythm, muscle contractions, and nerve signaling. Both high and low levels can cause dangerous heart rhythm problems.
Reference range: 3.5–5.0 mEq/L.
High potassium: Kidney disease, certain medications (ACE inhibitors, ARBs, NSAIDs), a blood sample that clumped in the lab (a common artifact). Low potassium: Diuretics, vomiting or diarrhea, inadequate dietary intake, excessive sweating.
Chloride
What it measures: Works with sodium to keep fluids balanced.
Why it matters: Chloride usually rises and falls with sodium. Your provider looks at it in context alongside other markers, not in isolation.
Reference range: 98–106 mEq/L.
Carbon Dioxide (CO2 / Bicarbonate)
What it measures: Reflects your body’s acid-base balance. Bicarbonate is the main buffer your body uses to keep blood pH in a very tight range.
Why it matters: Usually only concerning when combined with other abnormalities. On its own, a slightly off CO2 rarely changes management.
Reference range: 23–29 mEq/L.
Note for active individuals: Intense exercise produces lactic acid, which can temporarily lower your CO2 the next morning. This is generally transient and nothing to worry about.
Calcium
What it measures: Total calcium in your blood, including the active form and the portion bound to proteins.
Why it matters: Essential for bone health, heart function, nerve signaling, and blood clotting. Your body regulates calcium tightly using parathyroid hormone (PTH) and vitamin D, so even small deviations can be meaningful.
Reference range: 8.5–10.5 mg/dL.
High calcium: Primary hyperparathyroidism (the most common outpatient cause), cancer, excessive vitamin D supplementation, certain medications.
Low calcium: Vitamin D deficiency, kidney disease (kidneys activate vitamin D, so declining function means less calcium absorption), low albumin (can falsely lower total calcium while the active form is normal), magnesium deficiency.
A note on bone health: Serum calcium reflects your body’s calcium regulation system, not your bone density. You can have normal blood calcium while steadily losing bone. If bone health is a concern, a DEXA scan and vitamin D level give a more complete picture.
HOW THESE MARKERS TELL A STORY TOGETHER
The Kidney Function Triad: BUN + Creatinine + eGFR
Your provider looks at the ratio between BUN and creatinine:
- • Normal ratio (10:1 to 20:1): Both tracking proportionally. Consistent with baseline function.
- • High ratio (above 20:1) with normal creatinine: Often dehydration or high protein intake — kidneys are clearing creatinine fine, BUN is just elevated from other causes.
- • Both elevated: More concerning for actual kidney impairment or severe dehydration.
eGFR provides the summary. If BUN and creatinine are elevated but eGFR is above 60, muscle mass or dehydration is the likely driver. If eGFR stays below 60 on repeat testing, further evaluation is warranted.
The Dehydration Signature
Dehydration is the single most common reason for “abnormal” results on this panel. A dehydrated patient may show elevated BUN, creatinine, sodium, and chloride alongside a decreased eGFR — collectively mimicking early kidney disease when it is really just not enough water. Drinking normally before your draw is the simplest way to get accurate results.
WHAT AFFECTS THESE RESULTS
Hydration: The biggest variable. Even mild dehydration concentrates the blood and elevates BUN, creatinine, and sodium while lowering eGFR. Fasting for lipids does not mean skip water.
Protein intake: High-protein diets raise BUN. This is normal metabolism, not kidney damage. That said, very-high-protein intake (above 2.0 g/kg/day) in people with pre-existing CKD may accelerate progression.
Muscle mass: More muscle means more creatinine. The eGFR formula does not account for body composition, so expect higher-end creatinine if you carry significant muscle.
Creatine supplementation: Converted to creatinine in the body. Standard doses (3–5 g/day) can raise serum creatinine by 10–30% without any change in actual kidney function.
Medications: NSAIDs can reduce kidney blood flow. ACE inhibitors and ARBs can raise potassium and creatinine. Diuretics affect sodium, potassium, and chloride. Always provide a complete medication list.
Exercise timing: Intense training within 24–48 hours of a draw can transiently raise creatinine and potassium. When practical, skip the max-effort session the day before testing.
COMMON QUESTIONS
“My creatinine is high — does that mean kidney disease?”
Not necessarily. Creatinine is influenced by muscle mass, hydration, diet, supplements, and medications. In muscular, well-trained people, creatinine above the standard range may be completely normal. A repeat test with adequate hydration — and a cystatin C level if there is still doubt — can clarify.
“Does creatine supplementation affect my results?”
Yes. Creatine converts to creatinine, so standard doses raise serum creatinine without affecting actual kidney function — your eGFR will appear lower than it truly is. A cystatin C-based eGFR gives an independent estimate. Tell your clinician about creatine use so results are interpreted correctly.
“I was dehydrated before my test — could that matter?”
Absolutely. Dehydration can raise BUN, creatinine, sodium, and chloride while lowering eGFR — mimicking early kidney problems. Your clinician will likely recommend repeating the panel with proper hydration before drawing conclusions.
“How often should kidney function be checked?”
For healthy adults without risk factors, it is typically included in annual blood work. More frequent monitoring is recommended for people with CKD risk factors (diabetes, hypertension, cardiovascular disease, family history). Frequency increases with CKD stage — G3a may mean every 6–12 months, G4 every 1–3 months.
References
- 1. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). “Your Kidneys & How They Work.” NIH Publication, 2018.
- 2. Goyal A, Daneshpajouhnejad P, Engel K, et al. “Acute Kidney Injury.” StatPearls. StatPearls Publishing, 2024.
- 3. Centers for Disease Control and Prevention (CDC). “Chronic Kidney Disease in the United States, 2023.”
- 4. National Kidney Foundation (NKF). “About Chronic Kidney Disease.”
- 5. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. “KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.” Kidney International, 2024.
- 6. Hosten AO. “BUN and Creatinine.” In: Clinical Methods. 3rd ed. Butterworths, 1990.
- 7. Pagana KD, Pagana TJ. Mosby’s Manual of Diagnostic and Laboratory Tests. 7th ed. Elsevier, 2022.
- 8. Ko GJ, Obi Y, Tortorici AR, Kalantar-Zadeh K. “Dietary protein intake and chronic kidney disease.” Curr Opin Clin Nutr Metab Care, 2017.
- 9. Perrone RD, Madias NE, Levey AS. “Serum creatinine as an index of renal function.” Clinical Chemistry, 1992.
- 10. Pline KA, Smith CL. “The effect of creatine intake on renal function.” Annals of Pharmacotherapy, 2005.
- 11. Levey AS, Stevens LA, Schmid CH, et al. “A new equation to estimate glomerular filtration rate.” Annals of Internal Medicine, 2009.
- 12. Inker LA, Eneanya ND, Coresh J, et al. “New creatinine- and cystatin C-based equations to estimate GFR without race.” NEJM, 2021.
- 13. Stevens LA, Coresh J, Schmid CH, et al. “Estimating GFR using serum cystatin C alone and in combination with serum creatinine.” Am J Kidney Dis, 2008.
- 14. Adrogue HJ, Madias NE. “Hyponatremia.” NEJM, 2000.
- 15. Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. “Third International Exercise-Associated Hyponatremia Consensus.” Clin J Sport Med, 2015.
- 16. Palmer BF, Clegg DJ. “Physiology and pathophysiology of potassium homeostasis.” Adv Physiol Educ, 2016.
- 17. Berend K, van Hulsteijn LH, Gans ROB. “Chloride: the queen of electrolytes?” Eur J Intern Med, 2012.
- 18. Kraut JA, Madias NE. “Metabolic acidosis: pathophysiology, diagnosis and management.” Nat Rev Nephrol, 2010.
- 19. Raphael KL. “Metabolic acidosis in CKD: core curriculum 2019.” Am J Kidney Dis, 2019.
- 20. Brancaccio P, Maffulli N, Limongelli FM. “Creatine kinase monitoring in sport medicine.” British Medical Bulletin, 2007.
- 21. Goltzman D. “Approach to Hypercalcemia.” Endotext, 2019.
- 22. Walker MD, Silverberg SJ. “Primary hyperparathyroidism.” Nat Rev Endocrinol, 2018.
- 23. Kalantar-Zadeh K, Fouque D. “Nutritional management of chronic kidney disease.” NEJM, 2017.
- 24. Ungprasert P, Cheungpasitporn W, et al. “Individual NSAIDs and risk of acute kidney injury.” Eur J Intern Med, 2015.
This article is for informational purposes only and does not constitute medical advice. Blood work results should be interpreted by a qualified healthcare provider in the context of your complete medical history, medications, and clinical presentation.
UNDERSTAND YOUR NUMBERS
A kidney panel is only as useful as the interpretation behind it. Get your blood work reviewed by a provider who looks at the full picture — hydration, training, medications, and trends over time.
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