A 35-year-old man with a history of recurrent nephrolithiasis presents with acute flank pain, hematuria, and a rise in serum creatinine from 0.9 to 2.1 mg/dL over 6 hours. Urine output is 150 mL/day. Bedside ultrasound shows bilateral hydronephrosis. Which investigation is most appropriate to confirm the diagnosis and guide management?

Explanation

## Diagnosis of Postrenal Acute Kidney Injury (Obstructive AKI) ### Clinical Presentation The combination of: - Acute flank pain with hematuria - Bilateral hydronephrosis on ultrasound - Rapid rise in creatinine - History of nephrolithiasis ...strongly suggests **postrenal AKI due to bilateral ureteral obstruction**. ### Why Non-Contrast CT (CT KUB) Is the Gold Standard **Key Point:** Non-contrast CT (also called CT KUB or unenhanced helical CT) is the **gold standard for diagnosis and localization of urinary tract obstruction**, especially in the setting of suspected nephrolithiasis. **High-Yield:** CT KUB has: - **Sensitivity > 95%** for detecting stones - **Specificity > 98%** for stone composition and location - **No contrast required** (avoids further renal injury in AKI) - **Rapid acquisition** (guides urgent intervention) ### Why CT KUB Over Ultrasound? **Clinical Pearl:** While bedside ultrasound can detect hydronephrosis (confirming obstruction), it **cannot reliably identify the level or cause of obstruction** or detect radiolucent stones. CT KUB provides: - Exact stone location (proximal ureter, pelvic brim, distal ureter) - Stone size and density (guides treatment: medical expulsion vs. intervention) - Presence of complications (forniceal rupture, perinephric collection) ### Management Algorithm ```mermaid flowchart TD A[Bilateral hydronephrosis + AKI]:::outcome --> B[Non-contrast CT KUB]:::action B --> C{Stone location & size?}:::decision C -->|Proximal/mid ureter, < 6 mm| D[Medical expulsion therapy + hydration]:::action C -->|Proximal/mid ureter, > 6 mm| E[Urgent ureteral stent or percutaneous nephrostomy]:::action C -->|Distal ureter, any size| F[Consider ureteroscopy]:::action E --> G[Creatinine monitoring]:::action F --> G ``` ### Why Other Options Are Wrong **Serum uric acid & 24-hour urine uric acid:** - Relevant only if **uric acid nephropathy** is suspected (massive uric acid excretion, e.g., tumor lysis syndrome) - In this case, the clinical presentation (flank pain, hematuria, history of stones) points to mechanical obstruction, not metabolic AKI **Renal biopsy with immunofluorescence:** - Indicated for glomerulonephritis (proteinuria, dysmorphic RBCs, casts) - Not indicated in postrenal AKI; biopsy carries risk of bleeding in an already compromised kidney **FENa and urine osmolality:** - These tests assess **tubular function** and distinguish prerenal from intrinsic AKI - In postrenal AKI, urinary indices may be variable and are not diagnostic - Do not identify the cause of obstruction ### Why Contrast-Enhanced CT Is Avoided **Warning:** Contrast-enhanced CT should be avoided in acute AKI due to risk of contrast-induced nephrotoxicity. Non-contrast CT provides all necessary diagnostic information.