the kidney. Like all prostaglandins, PGE2 synthesis depends upon the cyclooxygenases. Aspirin and other NSAIDs are inhibitors of the cyclooxygenases. In the kidney, this inhibition results in decreased PGE2 concentration causing a reduction in blood flow. Because blood flow to the kidney first reaches the renal cortex (outside) and then the renal medulla (inside), the deeper structures of the kidney are most sensitive to decreased blood flow. Thus the innermost structures of the kidney, known as the renal papillae, are especially dependent on prostaglandin synthesis to maintain adequate blood flow. Inhibition of cyclooxygenases therefore rather selectively damages the renal papillae, increasing the risk of renal papillary necrosis. NSAIDs caused no adverse effects on renal function in healthy dogs subjected to anesthesia. Most healthy kidneys contain enough physiologic reserve to compensate for this NSAID-induced decrease in blood flow. However, those subjected to additional injury from phenacetin or paracetamol may progress to analgesic nephropathy. Phenacetin and paracetamol It is unclear how phenacetin induces injury to the kidney. Bach and Hardy have proposed that phenacetin's metabolites lead to lipid peroxidation that damages cells of the kidney. Paracetamol is the major metabolite of phenacetin and may contribute to kidney injury through a specific mechanism. In cells of the kidney, cyclooxygenases catalyse the conversion of paracetamol into N-acetyl-p-benzoquinoneimine (NAPQI). NAPQI depletes glutathione via non-enzymatic conjugation with glutathione, a naturally occurring antioxidant. With depletion of glutathione, cells of the kidney become particularly sensitive to oxidative damage. Diagnosis Diagnosis is traditionally based on the clinical findings above in combination with excessive analgesic use. Once suspected, analgesic nephropathy can be confirmed with relative accuracy using computed tomography (CT) imaging without contrast. One trial demonstrated that the appearance of papillary calcifications on CT imaging was 92% sensitive and 100% specific for the diagnosis of analgesic nephropathy. Treatment Treatment of analgesic nephropathy begins with the discontinuation of analgesics, which often halts the progression of the disease and may even result in normalization of kidney function. In Stage 5 chronic kidney disease patients renal replacement therapy may become necessary. History Analgesics are a class of medications widely used in the treatment of pain. They include aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs), as well as the antipyretics paracetamol (known as acetaminophen in the United States) and phenacetin. Introduced in the late 19th century, phenacetin was once a common component of mixed analgesics in parts of Europe, Australia, and the United States.