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SOP

Kidney Function Tests (KFTs) Panel

SOP for the renal panel: urea/BUN, creatinine, eGFR, and core electrolytes, with AKI and CKD decision limits and per-analyte procedures.

Last verified 1 month ago 6 min read Clinical Chemistry Renal Metabolic #electrolytes #egfr #bun #creatinine #uric-acid #urea #clinical chemistry #kft #kidney function #renal panel
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Document Control
Version
2.0
Effective
Aug 31, 2026
Reviewed
Aug 31, 2026
Department
Clinical Chemistry
Principle

Creatinine, produced from muscle creatine at a near-constant rate, is filtered by glomeruli and minimally reabsorbed, making it the primary analyte for estimating GFR. Urea/BUN reflects nitrogen balance and rises earlier in prerenal azotemia than in intrinsic renal disease. Sodium, potassium, chloride, and bicarbonate reflect tubular handling, electrolyte balance, and acid-base regulation; uric acid rises with reduced renal clearance in CKD. The ratio of BUN to creatinine helps distinguish prerenal from intrinsic or postrenal azotemia.

Standard Operating Procedure: Kidney Function Tests (KFTs) Panel

1. Purpose and Scope

To measure urea/BUN, creatinine, and the core electrolytes (sodium, potassium, chloride, bicarbonate/CO2, calcium, and uric acid) for the assessment of glomerular filtration, electrolyte balance, and acid-base status in the evaluation of acute kidney injury (AKI), chronic kidney disease (CKD), and fluid/electrolyte disorders.

2. Specimen Requirements

  • Type: Serum. Use a plain (no additive) tube or serum separator tube (SST).
  • Volume: Minimum 1.0 mL.
  • Hemolysis: Reject markedly hemolyzed samples for potassium and uric acid; hemolysis falsely raises potassium.
  • Stability: Separate serum from cells within 2 hours (critical for potassium). Creatinine and urea stable 24-48 h at 2-8°C; electrolytes stable 24 h refrigerated if cells are removed.
  • Timing: Random sampling is acceptable; note timing relative to dialysis or contrast administration when relevant.

3. Safety and Precautions

  • Standard PPE (gloves, lab coat, eye protection). All blood samples are potentially infectious.
  • Dispose of sharps and biohazard waste per laboratory policy.
  • Do not pipette by mouth. Use mechanical pipetting devices.

4. Equipment and Reagents

  • Automated chemistry analyzer (spectrophotometric and ion-selective electrode, ISE, capable).
  • Urea/BUN: Urease-glutamate dehydrogenase (GLDH) kinetic reagent, measuring NADH consumption at 340 nm.
  • Creatinine: Enzymatic creatinine reagent (creatininase cascade, preferred for specificity) or compensated Jaffe (alkaline picrate) kinetic method.
  • Sodium, Potassium, Chloride: Indirect (or direct) ion-selective electrodes.
  • Bicarbonate/CO2: Enzymatic or ISE method.
  • Calcium: Arsenazo III or o-cresolphthalein complexone (OCPC) colorimetric method.
  • Uric Acid: Uricase-peroxidase coupled (Trinder) or uricase end-point method.
  • Two levels of quality control material for each assay.

5. Per-Analyte Procedures

5.1 Urea / BUN Procedure

  1. Load the urease-GLDH reagent (urease, glutamate dehydrogenase, alpha-ketoglutarate, NADH) at 37°C.
  2. Piper 200 µL of reagent and 10 µL of serum; mix.
  3. Urease converts urea to ammonia and CO2; ammonia combines with alpha-ketoglutarate and NADH via GLDH.
  4. Monitor the decrease in absorbance at 340 nm (NADH consumption) over 3 min.
  5. Convert the reaction rate to mg/dL (BUN) using the calibrator; note that urea = BUN x 2.14.

5.2 Creatinine Procedure (Enzymatic)

  1. Load the enzymatic creatinine reagent (creatininase, creatinase, sarcosine oxidase, peroxidase).
  2. Piper 200 µL of reagent and 20 µL of serum; mix.
  3. Creatininase hydrolyzes creatinine to creatine; sarcosine oxidase generates hydrogen peroxide.
  4. Measure the colored quinoneimine product at 546 nm (Trinder reaction) or the kinetic change.
  5. Convert absorbance to mg/dL against a calibrator; report eGFR calculated by CKD-EPI.

5.3 Sodium (Na+) Procedure - Indirect ISE

  1. Prime the ISE module and calibrate with two level standards (low/high) and internal reference solution.
  2. Auto-dilute serum 1:30 in a high-ionic-strength buffer.
  3. Pass the diluted sample over the sodium-selective glass/electrode membrane.
  4. Measure the electrode potential against the reference electrode; apply the Nernst equation to derive Na+ concentration in mEq/L.
  5. Run two levels of control and confirm calibration drift is within tolerance before release.

5.4 Potassium (K+) Procedure - Indirect ISE

  1. Calibrate the potassium electrode with low and high standards.
  2. Auto-dilute serum 1:30 in buffer; pass over the valinomycin-based potassium-selective membrane.
  3. Record the potential difference against the reference electrode.
  4. Derive K+ concentration in mEq/L via the Nernst equation.
  5. Reject hemolyzed samples, which falsely elevate potassium; repeat a fresh specimen before reporting critical highs.

5.5 Chloride (Cl-) Procedure - Indirect ISE

  1. Calibrate the chloride electrode with low and high ion standards.
  2. Auto-dilute serum 1:30 in buffer; pass over the chloride-selective membrane.
  3. Measure the potential against the reference electrode.
  4. Derive Cl- concentration in mEq/L via the Nernst equation.
  5. Correlate with sodium and CO2 for anion-gap interpretation; run controls with each batch.

5.6 Bicarbonate / CO2 Procedure

  1. Load the enzymatic CO2 reagent (phosphoenolpyruvate carboxylase, malate dehydrogenase, NADH) at 37°C.
  2. Piper 200 µL of reagent and 10 µL of serum; mix. Total CO2 is released as bicarbonate and carbonic acid.
  3. Monitor the decrease in absorbance at 340 nm as NADH is oxidized.
  4. Convert the rate to mEq/L of bicarbonate (total CO2) using the calibrator.
  5. Analyze promptly; sample must be separated from cells soon to avoid loss of CO2 to the atmosphere.

5.7 Calcium Procedure (Arsenazo III)

  1. Load the Arsenazo III colorimetric reagent.
  2. Piper 200 µL of reagent and 10 µL of serum; mix.
  3. Calcium binds Arsenazo III forming a blue-purple chromophore.
  4. Measure absorbance at 650 nm against a blank.
  5. Read calcium in mg/dL from the calibration curve; adjust for albumin (ionized/serum-calcium interpretation) when clinical context requires.

5.8 Uric Acid Procedure (Uricase-Peroxidase)

  1. Load the uricase-peroxidase reagent (uricase, peroxidase, aminoantipyrine/DHBS chromogen).
  2. Piper 200 µL of reagent and 20 µL of serum; mix.
  3. Uricase oxidizes uric acid to allantoin with hydrogen peroxide production.
  4. Peroxidase couples H2O2 to a red quinoneimine dye.
  5. Measure absorbance at 520 nm and convert to mg/dL; reject grossly hemolyzed samples.

6. Common Calibration and Quality Control

  • Run two levels (normal and abnormal) of control for each assay daily, with every calibration, and with each new reagent lot.
  • Enforce Levey-Jennings rules; investigate trends or shifts before reporting patient results.
  • Participate in an external quality assessment (EQA) program for all panel analytes.
  • Verify control values fall within acceptable target ranges before patient reporting.

7. Decision limits

The eGFR and CKD/AKI categories below are interpretive decision thresholds and are stated as prose; the canonical adult reference intervals are displayed in the Reference Ranges panel.

  • CKD staging (KDIGO, by eGFR mL/min/1.73 m2): G1 >= 90 (normal, with kidney damage markers); G2 60-89 (mildly decreased); G3a 45-59; G3b 30-44; G4 15-29 (severely decreased); G5 < 15 (kidney failure). CKD is defined as eGFR < 60 or markers of kidney damage persisting >= 3 months.
  • AKI (KDIGO): increase in serum creatinine >= 0.3 mg/dL (26.5 µmol/L) within 48 hours, or >= 1.5x baseline within 7 days, or urine output < 0.5 mL/kg/h for 6 hours.
  • BUN/Creatinine ratio: a ratio > 20:1 typically reflects prerenal states (volume depletion, GI bleed, high-protein intake, catabolism); a ratio well below 10:1 may be seen with liver disease, low protein intake, or rhabdomyolysis-related creatinine rises.
  • Hyperkalemia: potassium > 5.5 mEq/L is hyperkalemia; potassium > 6.0 mEq/L with ECG changes or rapid rise is a medical emergency requiring urgent treatment.
  • eGFR caveats: eGFR is an estimate, not an exact GFR; interpret with caution in extremes of body composition, the elderly, and in acute settings where creatinine has not reached steady state.

Note on units: BUN is reported in mg/dL; urea concentration is approximately 2.14x the BUN value. Always verify which of the two the laboratory reports.

Reference Ranges

Browse all ranges

7–20 mg/dL

10–20 ratio

23–29 mEq/L

Calcium - Adult

Adult · Any

8.5–10.2 mg/dL

Critical values apply

Chloride - Adult

Adult · Any

98–107 mEq/L

0.74–1.35 mg/dL

0.59–1.04 mg/dL

Potassium - Adult

Adult · Any

3.5–5 mEq/L

Critical values apply

Sodium - Adult

Adult · Any

136–145 mEq/L

Critical values apply

3.4–7 mg/dL

2.4–6 mg/dL

Frequently Asked Questions

Creatinine is produced at a relatively constant rate from muscle metabolism and is freely filtered, making its serum level a more reliable steady-state estimate of glomerular filtration than urea, which varies with protein intake, catabolism, and hydration.
eGFR equations estimate rather than measure filtration. Accuracy falls with extremes of muscle mass, in the acutely ill where creatinine has not equilibrated, in pregnancy, and at very high or very low GFR values.
A ratio above 20:1 suggests prerenal azotemia (dehydration, reduced renal perfusion, or GI bleeding), whereas a low ratio is seen with intrinsic renal disease, liver disease, or malnutrition.