Hemolytic Uremic Syndrome (HUS)

01Pathophysiology

Hemolytic uremic syndrome is a thrombotic microangiopathy (TMA), meaning the core pathology involves damage to the endothelium of small blood vessels, leading to microthrombi formation in arterioles and capillaries.

In the typical (diarrhea-associated) form, the sequence begins with ingestion of Shiga toxin-producing E. coli, most commonly serotype O157:H7. The bacteria colonize the gut and produce Shiga toxin (Stx), which is absorbed through the inflamed intestinal mucosa into the bloodstream. Shiga toxin binds to globotriaosylceramide (Gb3) receptors, which are highly expressed on renal glomerular endothelial cells. This is why the kidney is the primary target organ. Once internalized, the toxin inhibits protein synthesis by cleaving ribosomal RNA, leading to endothelial cell injury and death.

The damaged endothelium triggers a cascade: platelet activation and aggregation within the microvasculature, formation of fibrin-rich microthrombi, and narrowing of the vessel lumen. Red blood cells are forced through these partially occluded vessels and are mechanically sheared, producing schistocytes (the hallmark fragmented red blood cells seen on peripheral smear). This mechanical destruction explains the microangiopathic hemolytic anemia. The consumption of platelets in the microthrombi produces thrombocytopenia. And the occlusion of renal microvasculature causes acute kidney injury (AKI) with oliguria or anuria.

This triad of microangiopathic hemolytic anemia + thrombocytopenia + acute kidney injury is the defining feature of HUS.

In the atypical form, the trigger is not an infectious toxin but rather dysregulated complement activation. Mutations in complement regulatory proteins (Factor H, Factor I, membrane cofactor protein/CD46) lead to unchecked activation of the alternative complement pathway on endothelial surfaces. The result is the same downstream process: endothelial injury, microthrombi, and the clinical triad. However, atypical HUS tends to be relapsing, carries a worse long-term renal prognosis, and does not follow a diarrheal prodrome.

02Classification and Clinical Manifestation

By System

03Diagnostic Workup

The best initial test when you suspect HUS is the combination of a CBC with peripheral blood smear and serum creatinine. The smear is the single most important test because it reveals schistocytes, which immediately point you toward a thrombotic microangiopathy. Combined with thrombocytopenia and an elevated creatinine, the clinical triad is established.

To confirm the etiology in typical HUS, obtain a stool culture on sorbitol-MacConkey agar (O157:H7 does not ferment sorbitol, distinguishing it from other E. coli) and a Shiga toxin assay. The toxin assay is more sensitive because stool cultures can be negative if obtained late in the illness. Always send both.

A critical step is ordering ADAMTS13 activity. This test separates HUS from thrombotic thrombocytopenic purpura (TTP). In HUS, ADAMTS13 activity is normal or only mildly reduced (> 10%). In TTP, ADAMTS13 is severely deficient (< 10%), typically due to autoantibodies. This distinction is not just academic: TTP requires urgent plasma exchange, while typical HUS does not.

Coagulation studies (PT, PTT, fibrinogen, D-dimer) should be checked to rule out disseminated intravascular coagulation (DIC). In HUS, these values are characteristically normal because the process is localized to the microvasculature, not a systemic consumptive coagulopathy. If you see prolonged PT/PTT and low fibrinogen, think DIC instead.

If the presentation is atypical (no diarrheal prodrome, older patient, relapsing course, or family history of renal failure), send complement levels (C3, C4) and refer for complement gene mutation analysis and anti-Factor H antibody testing. Low C3 with normal C4 is the classic pattern, reflecting alternative pathway activation.

04Management and Treatment

Typical HUS is managed with supportive care only. The most important early intervention is aggressive IV fluid resuscitation. Studies have shown that early volume expansion before the onset of oliguria is associated with less severe renal injury and a reduced need for dialysis. Once a child presents with the triad, careful fluid balance monitoring is essential to avoid volume overload.

The single most important "do not do" in typical HUS management is the avoidance of antibiotics. When STEC bacteria are lysed by antibiotics, they release large amounts of Shiga toxin, which paradoxically worsens the microangiopathic process. This has been demonstrated in clinical studies showing increased rates of HUS progression in children treated with antibiotics for STEC diarrhea. Exam questions will frequently present a child with bloody diarrhea and a positive stool culture for O157:H7, then ask about the next step. The answer is supportive care and monitoring, not antibiotics.

Platelet transfusion is another important "avoid" unless there is active life-threatening bleeding. Transfused platelets can be consumed in the ongoing microthrombi, fueling the disease process.

Dialysis is needed in approximately 50% of children with typical HUS and is initiated for standard indications: refractory hyperkalemia, severe metabolic acidosis, volume overload unresponsive to diuretics, or uremic symptoms (pericarditis, encephalopathy). Most children recover renal function within 2 to 3 weeks, though some may have residual proteinuria or hypertension.

For atypical HUS, the treatment of choice is eculizumab, a humanized monoclonal antibody that binds complement protein C5 and prevents formation of the membrane attack complex (C5b-9). By blocking terminal complement activation, eculizumab halts the endothelial damage driving the disease. The critical safety concern with eculizumab is the increased risk of Neisseria meningitidis infection because the terminal complement pathway is essential for defense against encapsulated organisms, particularly Neisseria. Patients must receive the meningococcal vaccine (covering serogroups A, C, W, Y, and B) at least 2 weeks before starting therapy. If treatment cannot be delayed, give prophylactic ciprofloxacin or penicillin until the vaccine takes effect.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The number one trap with HUS is the antibiotic question. Vignettes will describe a child with bloody diarrhea and a stool culture growing E. coli O157:H7, then ask what to do next. Many students reflexively choose antibiotics for a bacterial diarrheal illness, but this is the wrong answer. Antibiotics increase Shiga toxin release and worsen HUS. The correct answer is supportive care with monitoring for renal complications.

A second common trap is confusing HUS with TTP. Both are thrombotic microangiopathies with overlapping features, and the vignette may include neurologic symptoms in a HUS patient (which can occur in ~20 to 25% of cases). The discriminator is ADAMTS13 activity: normal in HUS, severely deficient in TTP. Additionally, HUS predominantly affects young children and is preceded by bloody diarrhea, while TTP typically affects adults and presents with predominant neurologic dysfunction.

A third pitfall involves platelet transfusion. Students often see a dangerously low platelet count and want to transfuse. In HUS, platelet transfusion is contraindicated unless there is active life-threatening hemorrhage because it provides fuel for ongoing microvascular thrombosis.

Students should also remember the link between eculizumab and meningococcal vaccination. If a vignette describes atypical HUS being treated with eculizumab, the follow-up question will often ask about necessary vaccinations or infection prophylaxis.

Finally, do not confuse the normal coagulation studies in HUS with the abnormal coagulation studies in DIC. This is a frequently tested differentiator. Both conditions produce schistocytes, but only DIC causes a consumptive coagulopathy with prolonged PT/PTT and low fibrinogen.

The core competency being tested is your ability to recognize the clinical triad (microangiopathic hemolytic anemia, thrombocytopenia, acute kidney injury), trace it back to its etiology through the correct diagnostic workup, and apply the appropriate management while avoiding the interventions that worsen the disease.