What does a blood urea nitrogen of 22 mg/dL, BUN‑to‑creatinine ratio of 23:1, and estimated glomerular filtration rate of 110 mL/min/1.73 m² indicate in a 31‑year‑old male?

Interpretation of Laboratory Values in a 31-Year-Old Male

These laboratory values are entirely normal and require no intervention. The BUN of 22 mg/dL falls within the normal reference range (typically 7–20 mg/dL, though up to 22 mg/dL is often considered acceptable), the BUN-to-creatinine ratio of 23:1 is within normal limits (normal range 10:1 to 20:1, with up to 23:1 still considered physiologic), and the GFR of 110 mL/min/1.73 m² indicates excellent kidney function well above the threshold for any stage of chronic kidney disease. 1

Normal Kidney Function Assessment

• The estimated GFR of 110 mL/min/1.73 m² is substantially above the threshold for any kidney disease, which begins at <90 mL/min/1.73 m² in the presence of kidney damage markers, or <60 mL/min/1.73 m² for chronic kidney disease staging. 1

• This GFR value indicates hyperfiltration or robust normal kidney function typical for a healthy 31-year-old male, and is not associated with any adverse renal outcomes. 1

• The KDIGO 2024 guidelines recommend reporting eGFR values <60 mL/min/1.73 m² as flagged low; values ≥60 mL/min/1.73 m² like this one require no special notation. 1

BUN and BUN-to-Creatinine Ratio Interpretation

• The BUN of 22 mg/dL is at the upper limit of normal but does not indicate pathology in the context of normal kidney function (GFR 110 mL/min/1.73 m²). 2

• The BUN-to-creatinine ratio of 23:1 is slightly above the typical range of 10:1 to 20:1 but does not reach the threshold of >20:1 that would strongly suggest prerenal azotemia from dehydration or volume depletion. 3, 4

• A ratio of 23:1 may reflect mild dehydration, higher dietary protein intake, or normal physiologic variation, none of which are concerning in a young healthy adult with excellent kidney function. 3

• In dehydration, BUN rises disproportionately to creatinine because 40–50% of filtered urea is reabsorbed in the proximal tubule (paralleling sodium and water reabsorption), whereas creatinine is not significantly reabsorbed. 3

Clinical Context and Significance

• No intervention is needed because all three values indicate normal to excellent kidney function without evidence of acute or chronic kidney disease. 1, 2

• The slightly elevated BUN-to-creatinine ratio does not warrant concern in the absence of clinical symptoms (orthostatic hypotension, decreased skin turgor, dry mucous membranes) or other laboratory abnormalities. 3, 4

• If mild dehydration is suspected clinically, simple oral rehydration would be expected to normalize the BUN within 24–48 hours, though this is not medically necessary given the overall normal kidney function. 4

When to Reassess

• Routine follow-up is not required based on these laboratory values alone in an asymptomatic 31-year-old. 1

• Repeat testing would only be indicated if clinical symptoms develop (decreased urine output, edema, hypertension) or if there are risk factors for kidney disease such as diabetes, hypertension, or family history of kidney disease. 2, 4

• Annual screening with urinalysis and eGFR is reasonable if diabetes or hypertension develops in the future, but is not indicated based on these normal values. 4

Common Pitfalls to Avoid

• Do not misinterpret the slightly elevated BUN-to-creatinine ratio as indicating kidney disease when the GFR is clearly normal at 110 mL/min/1.73 m². 2

• Do not order unnecessary imaging or nephrology referral for these normal values; nephrology referral is indicated only for eGFR <30 mL/min/1.73 m², rapidly progressive kidney disease, or uncertainty about etiology of kidney disease. 4

• Remember that serum creatinine alone can underestimate kidney dysfunction in patients with low muscle mass, but the calculated eGFR already accounts for age, sex, and race, making it more reliable than creatinine alone. 1, 2