IM10.12-16 | Renal Failure Diagnostic Testing — Summary & Reflection

KEY TAKEAWAYS

Diagnostic testing in renal failure is hypothesis-driven, not blanket. The minimum panel covers creatinine/eGFR, electrolytes, urea, FBC, urinalysis with microscopy, and renal ultrasound.

Key derived indices: FENa <1% = pre-renal (intact tubular Na conservation); FENa >2% = ATN (tubular dysfunction). Caveat: FENa unreliable on diuretics — use FEUrea <35% for pre-renal in this setting. Urea:creatinine ratio >20:1 = pre-renal or GI bleed.

Anion gap metabolic acidosis: Normal AG (hyperchloraemic) = early CKD/RTA; High AG (MUDPILES) = advanced CKD/AKI, lactic acidosis, DKA.

ECG hyperkalaemia sequence: Peaked T-waves (K 5.5–6.0) → PR prolongation + P-wave flattening (K 6.0–7.0) → QRS widening (K 6.5–7.5) → sine-wave (K >7.5) → VF. QRS widening = immediate calcium gluconate.

Urinalysis casts: Granular/muddy-brown = ATN; red cell casts = GN; white cell casts = AIN/pyelonephritis; waxy/broad casts = advanced CKD.

Ultrasound: Small hyperechoic kidneys = CKD; hydronephrosis = obstruction; normal/enlarged = AKI or infiltrative. ABG: metabolic acidosis with respiratory compensation (Winter formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2); deviation from expected = mixed disorder.

Procedural: ABG from radial artery (Allen's test first, 45° angle, 5 min pressure); IV cannula (18G for fluids, 20G routine; avoid AV fistula arm in dialysis patients).

REFLECT

Return to the opening hook — the patient with creatinine 3.8, potassium 6.8, bicarbonate 14, and a sine-wave ECG. With the knowledge from this module, you can now systematically answer every question the case posed: the FENa (from urine Na, urine Cr, serum Na, serum Cr) will tell you whether the tubules are intact (pre-renal) or injured (ATN); the granular casts on urinalysis already confirm ATN; the bicarbonate of 14 gives you an anion gap to calculate and places you in the metabolic acidosis algorithm; the ECG sine-wave pattern with QRS of 130 ms mandates immediate calcium gluconate before the potassium level even returns. The diagnostic workup is not an abstract academic exercise — each result you interpret correctly in the first hour of management is a life saved or a complication prevented. Think about which of these investigations you have performed or observed on a real patient. What finding surprised you? What result triggered an immediate management change? That connection between the investigation and the action is what this module aims to build.