The nephrotoxic kidney is the one injury you can most often prevent, because you usually wrote the order that caused it. Every nephrotoxin has a signature — a mechanism, a pattern, a timing — and knowing the signature lets you both recognise the culprit and stop the next one. The chapter's recurring lesson is that with drug injury, the medication list is the test, prevention is the treatment, and contrast deserves a more measured fear than it has historically received.
Many mechanisms, predictable patterns
Nephrotoxins do not all injure the kidney the same way, and the mechanism predicts what you will see. Direct tubular toxins produce ATN with granular casts. Allergic interstitial reactions produce acute interstitial nephritis, with white-cell casts and sometimes a rash — the subject of the next chapter. Some drugs precipitate as crystals and obstruct the tubule; others act haemodynamically, constricting the afferent or blocking the efferent arteriole; a few trigger thrombotic microangiopathy; and a handful cause osmotic nephrosis. The practical move is to ask, for any suspect drug, which of these it is known to do — because that tells you where to look in the urine and how to confirm it.
Why the proximal tubule takes the hit
Most direct toxins injure the proximal tubule, and for a reason: it is the segment that reabsorbs and concentrates, so it accumulates whatever it handles. Aminoglycosides are taken up at the brush border by megalin and accumulate in lysosomes; cisplatin and tenofovir enter through organic-cation and organic-anion transporters. Once inside, they injure mitochondria, generate oxidative stress, and trigger apoptosis. The tubular cell, in effect, poisons itself by doing its job of uptake. This is why dose and duration matter so much, why volume depletion — which raises the intratubular concentration — worsens toxicity, and why a damaged tubule is more vulnerable to the next hit.
Contrast: a measured fear
Iodinated contrast injures the kidney by three combined routes: it constricts the medullary microcirculation through endothelin and adenosine, adding hypoxia to the vulnerable outer medulla; it is directly toxic to tubular cells through osmotic and oxidative effects; and its viscosity slows tubular flow. That is real. But the story has been overstated. Much of what was historically labelled 'contrast-induced nephropathy' was an artefact of attribution — sick patients who received contrast and then developed AKI from their underlying illness, with the dye taking the blame. Better-controlled studies, including propensity-matched cohorts, show that the AKI risk of modern low- and iso-osmolar contrast is considerably lower than the older literature suggested, particularly for intravenous contrast and in patients with an eGFR above 30. The field now separates contrast-associated AKI — a mere temporal association — from contrast-induced AKI, which is genuinely caused. The clinical consequence is balance: take precautions in the truly high-risk, but do not withhold a contrast study the patient genuinely needs out of an exaggerated fear, because a missed diagnosis harms too.
Pigment: heme as a toxin
Myoglobin from injured muscle and free haemoglobin from haemolysis are nephrotoxins in their own right. They injure through three mechanisms: pigment casts obstruct the tubule, especially in acidic urine; the heme molecule releases free iron that drives oxidative tubular injury; and the pigments provoke renal vasoconstriction. Volume depletion magnifies all three by concentrating the pigment and lowering flow, which is why the cornerstone of management is early, generous isotonic fluid to dilute the pigment and restore flow. The detailed management of rhabdomyolysis and the role — and limits — of bicarbonate and mannitol belong to Chapter 12; here the point is to recognise pigment as a nephrotoxic ATN and to start fluids early.
Calcineurin inhibitors and the haemodynamic toxins
Not all drug injury is structural. Calcineurin inhibitors — ciclosporin and tacrolimus — cause a dose-dependent afferent arteriolar vasoconstriction that lowers GFR functionally and reverses when the level falls, alongside a chronic toxicity of thrombotic microangiopathy and fibrosis. NSAIDs, ACE inhibitors, and ARBs act on the autoregulatory arterioles, as Chapter 2 described. Recognising the haemodynamic toxins matters because their acute effect is reversible: lower the dose or stop the drug and the GFR recovers, without the days-to-weeks course of structural ATN.
Combinations and the second hit
Risk is rarely from one drug in isolation. The vulnerable patient — with CKD, diabetes, volume depletion, sepsis, or age — receiving several nephrotoxins at once is where injury happens. Vancomycin combined with piperacillin-tazobactam is associated with more AKI than vancomycin with alternative beta-lactams, though part of that signal may reflect piperacillin-tazobactam inhibiting tubular creatinine secretion — a pseudo-AKI overlap worth remembering from Chapter 1. The NSAID-plus-ACE-inhibitor-plus-diuretic 'triple whammy' is the haemodynamic equivalent. The preventive instinct is to avoid stacking nephrotoxins on an at-risk kidney, and to treat each new agent on an injured kidney as a potential second hit.
Prevention is the treatment
Because established nephrotoxic ATN is managed exactly like the ATN of the previous chapter — supportively, with no specific cure — the real therapeutics are preventive. Identify the at-risk patient. Use the lowest effective dose for the shortest duration, and monitor levels where you can: aminoglycosides given once daily and dosed by levels are less toxic than divided regimens, and vancomycin is best guided by exposure rather than trough alone. Choose the liposomal amphotericin formulation and load sodium; hydrate and supplement magnesium with cisplatin. Maintain euvolaemia, since a dry tubule concentrates the toxin. For contrast, give isotonic crystalloid volume expansion to those at risk, minimise the contrast volume, use a low- or iso-osmolar agent, and hold other nephrotoxins — but spend no effort on N-acetylcysteine, which does not work, and do not assume bicarbonate beats saline, because it does not. And in every AKI, reconcile the medication list, because the cause is so often already written on the chart.
