CMN Nefrología common mistakes
CMN Nefrología · Understand the traps and how to avoid them.
Calculates the fractional excretion of sodium (FENa) or uses urine sodium to classify acute kidney injury without first confirming the patient is not on diuretics and that the sample was collected before any intervention.
FENa is taught as the discriminator between prerenal and intrinsic AKI, so candidates apply it reflexively and forget the diuretic caveat that invalidates the cutoff of 1%.
What to do instead
Before interpreting FENa, state explicitly whether diuretics were given; if so, switch to the FEUrea (<35% suggests prerenal) or the urine sodium-to-creatinine ratio, and note that contrast and CKD also shift the values.
Estimates glomerular filtration rate from serum creatinine alone in patients with low muscle mass, amputation, cachexia, or very high muscle mass, and treats the eGFR as accurate rather than as an estimate with a stated caveat.
Creatinine-based equations are memorized as definitive numbers, and the muscle-mass dependency of creatinine generation is underweighted.
What to do instead
For each creatinine-based eGFR, add a one-line caveat on non-renal determinants of creatinine (muscle mass, diet, drugs blocking tubular secretion) and, when it changes management, confirm with cystatin C-based eGFR or a measured clearance.
Reads an arterial blood gas in a single pass, labeling the primary disorder without formally checking the compensatory response and the anion gap, and without recalculating the gap for hypoalbuminemia.
Pattern recognition on pH and PaCO2 feels sufficient, and the stepwise algorithm (primary disorder, expected compensation, gap, delta-delta, osmolar gap) is skipped under time pressure.
What to do instead
Use a fixed written sequence for every gas: pH, primary disorder, expected compensation formula, anion gap corrected for albumin, delta-delta, and osmolar gap when indicated; write each step's result before concluding.
Classifies a nephrotic syndrome by histology name without first excluding secondary causes and without linking the serologies and clinical clues (age, complement level, systemic symptoms) to the likely diagnosis.
Memorized associations between light microscopy patterns and diseases dominate, while the diagnostic workup that precedes biopsy interpretation is treated as background.
What to do instead
For any nephrotic case, build the differential from age, complement level, and systemic features first, then state which serologies (ANA, anti-PLA2R, hepatitis, cryoglobulins, SPEP/free light chains) must be sent before or alongside biopsy.
Manages hyperkalemia by giving calcium, insulin/glucose, and beta-agonist without distinguishing which interventions shift potassium intracellularly versus remove it from the body, and without arranging definitive removal.
Therapies are memorized as a list rather than by mechanism and onset, so the temporary nature of shifting agents is lost.
What to do instead
For every hyperkalemia question, tabulate each therapy by mechanism (membrane stabilization, transcellular shift, bodily removal), onset, and duration, and always state the definitive removal plan (diuresis, binder, dialysis) alongside the temporizing measures.
Selects a dialysis modality or transplant workup step without checking the patient-specific contraindications and prerequisites (vascular access feasibility, hemodynamic stability, peritoneal membrane integrity, cardiac status, adherence, living-donor vs deceased-donor timeline).
Modality choice is reduced to a preference ranking, and the contraindication lists for each modality are recalled incompletely.
What to do instead
For each modality question, write the patient's constraints first (hemodynamics, abdomen, access vessels, home support, residual renal function) and match them against a checklist of absolute and relative contraindications before choosing.
Treats a positive urine culture as proof of urinary tract infection without integrating symptoms, pyuria, colony count, and the possibility of asymptomatic bacteriuria or contamination.
Culture positivity is equated with infection, and the distinction between asymptomatic bacteriuria and true UTI, especially in elderly or catheterized patients, is blurred.
What to do instead
For every culture result, require a symptom-pyuria-culture triad before diagnosing UTI, and explicitly state when antibiotics are not indicated (asymptomatic bacteriuria outside pregnancy or pre-urologic instrumentation).
Attributes a stone or obstruction to a single composition or cause without checking urine pH, stone analysis, and the metabolic workup that determines recurrence prevention.
Stone types are memorized by appearance and associated conditions, but the pH-dependent solubility and the indicated 24-hour urine studies are not connected to management.
What to do instead
For each stone scenario, link composition to urine pH and to the specific 24-hour urine parameters to check, then state the targeted preventive measure rather than a generic 'increase fluids' answer.
Doses or withholds renally cleared drugs and nephrotoxins using a remembered rule of thumb instead of adjusting for the measured or estimated clearance and the drug's dialyzability when applicable.
Pharmacokinetic principles (volume of distribution, protein binding, dialyzability, active metabolites) are underapplied, and dose adjustment is treated as a single correction factor.
What to do instead
For any drug in renal impairment, state the clearance used, the fraction renally excreted, whether metabolites are active, and whether the drug is dialyzable; adjust dose or interval explicitly and name the monitoring parameter.
Interprets pediatric renal findings using adult reference values and adult thresholds for blood pressure, creatinine, and fluid/electrolyte handling without correcting for age and body size.
Adult nephrology frames dominate studying, and the age-dependent maturation of GFR, tubular function, and blood pressure norms is underweighted.
What to do instead
For every pediatric item, first state the age-specific normal range or percentile being used (eGFR by Schwartz formula, BP percentiles by age/height/sex) before interpreting the abnormality.