Focal nephrogenic necrosis is caused by acute renal failure. The clinical tests that need to be done are: 1. Blood tests There is mild to moderate anemia. Blood creatinine and urea nitrogen rise progressively, and rise faster if combined with hypercatabolism and rhabdomyolysis caused by rhabdomyolysis, and hyperkalemia (>5.5mmol/l) may occur. Blood pH is often below 7.35, and HCO3- levels tend to be mildly to moderately reduced. The blood sodium concentration is normal or low, and there may be lower blood calcium and higher blood phosphorus. 2. Urine examination (1) Urine volume changes: little or no urine often indicates ATN. (2) Routine urine examination: turbid appearance and dark urine color. Urine protein is mostly – to +, often mainly medium and small molecule protein. The urine sediment can be seen as renal tubular epithelial cells, epithelial cell tubular type and granular tubular type, and a few red and white cells, etc. The urine specific gravity is often below 1.015. (3) Urine osmolality is lower than 350mOsm/kg, and the ratio of urine to blood osmolality is lower than 1.1. Urine sodium is increased due to reduced sodium reabsorption by renal tubules, mostly in the range of 20-60mmol/l; the ratio of urine creatinine to blood creatinine is reduced, often below 20; the ratio of urine urea nitrogen to blood urea nitrogen is reduced, often below 3; the renal failure index is often greater than 1; the sodium excretion fraction is often greater than 1. 3. Imaging In acute renal failure, the kidney volume often increases and the renal cortex may be thickened, while in chronic renal failure, the kidney volume often decreases and the renal cortex becomes thinner. Ultrasonography is also useful to identify the presence of posterior renal obstruction. Bilateral dilatation of the upper ureter or bilateral hydronephrosis can be seen in the case of upper urinary tract obstruction and bladder urinary retention in the case of lower urinary tract obstruction. Abdominal X-ray plain film, intravenous or retrograde pyelogram, CT or magnetic resonance imaging are usually helpful to find the exact cause of suspected urinary tract obstruction. 4. Renal biopsy is an important diagnostic tool. For patients with ATN with typical clinical manifestations, renal biopsy is usually not necessary. For patients with clinical manifestations consistent with ATN, but with oliguria for more than 2 weeks or with unknown etiology, and with renal function not recovered in 3-6 weeks, and with other serious renal parenchymal diseases leading to acute kidney injury under clinical consideration, renal biopsy should be performed as early as possible to clarify the etiology and diagnosis at an early stage.