Renal Function Test (RFT) — Don't Stop at Cr, Use eGFR + Urine Protein

Renal Function Test (RFT) — Don't Stop at Cr, Use eGFR + Urine Protein

Which lab should you check first before giving a nephrotoxic drug, ordering a contrast study, or running fluids in an elderly patient? It's the RFT panel: BUN, Cr, and eGFR.

The clinical question is simple — "How much can this kidney tolerate?" But there's one trap new nurses fall into constantly: stopping the evaluation at Cr. The real answer in clinical practice is eGFR + Urine Protein (two main markers), and the same number means different things in different patients.

■ Kidney = a 100,000-glomeruli water filter

The kidney is essentially a filter that purifies blood and makes urine. The actual filtering elements are the glomeruli. Each kidney has about 100,000 glomeruli, and as each one fails, kidney function drops proportionally.

What the RFT 3-panel really measures is the state of these glomeruli. BUN and Cr are waste products that remain in the blood when the glomeruli can't filter them out, and eGFR is a direct estimate of the glomeruli's filtering capacity.

■ Normal ranges and clinical meaning

BUN normal range is 8~23 mg/dL. When protein breaks down, the liver converts it to urea, and the kidneys excrete it. When the kidneys are damaged, urea builds up in the blood.

Cr normal range is 0.5~1.2 mg/dL, and it's the waste product from muscle metabolism. Clinically, Cr is trusted more than BUN — because it's less influenced by non-renal factors and reflects true kidney function more reliably.

eGFR normal is ≥90 mL/min/1.73m². It estimates creatinine clearance with age, sex, and body surface area corrections, so it shows actual filtering capacity most accurately.

■ BUN — Protein Waste (Never Interpret Alone)

BUN is dangerous to read alone because many non-renal conditions can raise it.

Dehydration is the classic example. When blood is concentrated, BUN rises. High-protein diets, GI bleeding (protein absorbed in the gut), stress, and burns can all push BUN up. Liver disease and malnutrition, on the other hand, often lower BUN — because the body can't metabolize protein properly.

So don't jump from "BUN is up" to "kidney damage." Always check Cr and eGFR together.

■ Cr — The Clinical Core (Still Has Limits)

Cr is more reliable than BUN because non-renal influences are smaller and daily variation is minimal, making trend tracking easier.

But Cr has its limits. It depends on muscle mass — men usually run higher than women, younger people higher than older ones. Heavy exercise or a high-protein meal before the test can temporarily raise Cr. Patients on H2 blockers may show falsely elevated Cr.

The most common pitfall for new nurses is the elderly or low-muscle-mass patient. Their baseline Cr is naturally low, so kidney damage may not push it above the normal range. Don't be reassured by "Cr is normal" in these patients.

A typical clinical scenario: a patient without a known kidney disease shows rising Cr with dehydration signs. The first-line response is hydration — normal saline or similar isotonic fluids to support kidney function while correcting dehydration. Persistent dehydration can progress to acute kidney injury (AKI), so the response needs to be prompt.

■ eGFR — The True Kidney Function Marker

eGFR estimates glomerular filtration capacity. Three clinical numbers to memorize:

First, if Cr doubles, kidney function is estimated to have dropped ~50%. So Cr going from 1.0 to 2.0 means roughly half the kidney is gone. Second, Cr ≥10 typically prompts dialysis consideration. Third, after age 20–30, eGFR naturally declines about 1 mL/min/1.73m² per year.

That natural decline creates the most confusing trap. A 60-year-old woman with eGFR 70 — that's age-related decline, not chronic kidney disease. But the same eGFR 70 in a 20-year-old man should strongly raise suspicion of CKD or AKI.

In short, eGFR isn't read in isolation — interpret it with age, sex, and the patient's prior trend. That's why the book emphasizes: even when BUN and Cr are in range, a falling eGFR may indicate ongoing kidney injury.

■ CKD Staging — Based on eGFR

Chronic kidney disease is staged 1 through 5 by eGFR, with stage 3 further split into 3a and 3b in clinical practice.

Stage 1: eGFR ≥90 — normal, routine monitoring. Stage 2: 60–89 — mild decrease, start renoprotective medications. Stage 3a: 45–59 — moderate decrease, start dietary restriction. Stage 3b: 30–44 — moderate-to-severe decrease, specialist consultation. Stage 4: 15–29 — severe damage, prepare for dialysis. Stage 5: <15 — kidney failure, dialysis or transplant.

The critical threshold is eGFR 60. Below this line, the diagnosis is CKD and management intensifies.

■ 4 Causes of Kidney Damage + Extra Monitoring in Kidney Failure

The book lists four main causes:

First, nephrotoxic medications — aminoglycoside antibiotics and NSAIDs are the classic examples. Review for discontinuation, look for alternatives, and optimize fluid status.

Second, infection — UTI or sepsis progressing to the kidney. Culture, antibiotics, and fluid resuscitation are the basics.

Third, chronic hypertension, which damages renal vasculature. Blood pressure control comes first.

Fourth, diabetes — the leading cause of CKD in Korea. Glycemic control is the priority.

For patients in kidney failure, monitoring goes beyond renal function. Potassium ≥6.0 brings cardiac arrest risk and may need potassium binders. Hgb ≤12 often reflects EPO deficiency anemia — EPO injections may be considered. Albumin ≤3.5 risks ascites and pleural effusion, requiring aggressive nutritional support.

■ Kidney Evaluation = eGFR + Urine Protein (2 Main Markers)

The most important point — never stop at eGFR. The proper kidney assessment combines eGFR and urine protein, the two main markers.

Why urine protein matters: a healthy kidney doesn't let proteins through the glomerulus — they're too large. So protein appearing in urine is a first-line signal of glomerular damage. Even with eGFR in the normal range, a positive urine protein should raise suspicion of early CKD.

Testing rules also matter. A single positive doesn't confirm anything — it can be transient, so at least 3 repeat tests are needed. In diabetic patients, don't rely on HbA1c alone — track eGFR and urine protein regularly.

Cystatin C is a supportive marker. Since it's released by all nucleated cells, it isn't affected by muscle mass or diet, so it picks up early kidney decline more sensitively. When eGFR drifts around the 60 boundary (62 one day, 58 the next), Cystatin C helps clarify whether stage 3 has actually been reached. Downsides: it's more expensive than standard Cr testing and usually only available at tertiary hospitals.

Finally, CKD high-risk groups need scheduled screening. Patients with diabetes, hypertension, age 60+, or long-term NSAIDs use should get eGFR and urine protein checked at least once a year.

■ Summary

The RFT 3-panel must be read as a set. BUN is too influenced by non-renal factors to interpret alone, Cr is the clinical core but varies with muscle mass and diet, and eGFR is the truest function marker but still insufficient by itself. The correct answer is eGFR + urine protein — the two main markers.

The flow new nurses must internalize: Cr doubling means ~50% function loss. eGFR <60 diagnoses CKD; eGFR <15 calls for dialysis. The same number means different things in different patients (60F with 70 = normal, 20M with 70 = suspicious). Cr rising with dehydration → hydration is the first-line response.

Checking RFT before nephrotoxic drugs, contrast media, and fluid decisions — that's the starting point of patient safety.

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