What are the causes and differential diagnosis of acute kidney injury (AKI) in pregnancy?

Causes and Differential Diagnosis of AKI in Pregnancy

AKI in pregnancy must be systematically categorized into prerenal, intrinsic renal, and postrenal causes, with pregnancy-specific conditions (preeclampsia/HELLP, acute fatty liver of pregnancy, and thrombotic microangiopathies) representing the most critical and challenging diagnoses requiring immediate differentiation to prevent maternal and fetal mortality. 1, 2

Timing-Based Differential Approach

The causes of pregnancy-associated AKI (PrAKI) vary dramatically by trimester, which provides the most practical framework for differential diagnosis:

First Trimester Causes

• Septic abortion (historically 9% of all AKI cases, now declining to 5%) 3
• Hyperemesis gravidarum (volume depletion from severe vomiting)
• Post-abortal sepsis

Third Trimester and Postpartum Causes (Most Common Period)

Pregnancy-specific conditions (these are the diagnostic challenges):

• Preeclampsia/eclampsia - most common cause in late third trimester 3
• HELLP syndrome (Hemolysis, Elevated Liver enzymes, Low Platelets)
• Acute fatty liver of pregnancy (AFLP)
• Pregnancy-associated thrombotic microangiopathies (P-TMA):
  • Atypical hemolytic uremic syndrome (aHUS)
  • Thrombotic thrombocytopenic purpura (TTP)

Non-pregnancy-specific conditions:

• Puerperal sepsis 3
• Postpartum hemorrhage 3
• Volume depletion
• Pyelonephritis 4

Standard AKI Classification Framework

Beyond pregnancy-specific timing, apply the traditional prerenal-intrinsic-postrenal framework 5:

Prerenal Causes (Functional)

• Hypotension/hypovolemia from hemorrhage, hyperemesis, or sepsis
• Decreased cardiac output
• Renal artery occlusion (rare)

Intrinsic Renal Causes (Structural)

• Acute tubular necrosis (ATN) - accounts for approximately 35% of AKI in cirrhosis studies, likely similar in pregnancy 6
• Acute glomerulonephritis
• Acute interstitial nephritis (drug-induced, rare)
• Lupus nephritis (in patients with known SLE) 2
• Renal infection/infiltration

Postrenal Causes (Obstruction)

• Kidney stones 2
• Iatrogenic ureteral injury during delivery 2
• Bladder/urethral obstruction (very uncommon, <3% of all AKI) 5

Critical Diagnostic Challenge: The Thrombotic Microangiopathy Overlap

The most diagnostically challenging scenario is differentiating preeclampsia/HELLP, AFLP, and P-TMA (aHUS/TTP), as all three share overlapping features of thrombotic microangiopathy. 3, 4 This distinction is clinically critical because:

• HELLP and AFLP typically improve after delivery of the fetus
• P-TMA (aHUS/TTP) requires plasma exchange as first-line treatment 3
• aHUS may benefit from eculizumab therapy 4

Key Differentiating Features:

Clinical criteria alone are often insufficient 3, 4. Look for:

• Timing: HELLP/preeclampsia typically occur before delivery; aHUS/TTP more commonly postpartum

• Laboratory patterns:

  • AFLP: severe hypoglycemia, coagulopathy, elevated ammonia
  • TTP: severe thrombocytopenia (<30,000), neurologic symptoms, ADAMTS13 activity <10%
  • aHUS: complement dysregulation, may have genetic mutations
  • HELLP: hemolysis, elevated liver enzymes, platelets 50,000-100,000

• Response to delivery: Lack of improvement 48-72 hours post-delivery suggests P-TMA rather than HELLP/AFLP 3

Diagnostic Evaluation Strategy

Urinalysis is the critical first step to differentiate functional from structural causes 6, 7:

• Absence of hematuria, proteinuria (>500 mg/day), or abnormal casts suggests functional/prerenal causes
• Presence of red blood cells (>50/HPF), significant proteinuria, or cellular casts indicates intrinsic renal disease 6, 8

Laboratory workup must include 2:

• Serial serum creatinine (increase ≥0.3 mg/dL within 48 hours or ≥1.5× baseline defines AKI) 5
• Complete blood count with peripheral smear (for schistocytes)
• Liver function tests
• Coagulation studies
• LDH, haptoglobin (hemolysis markers)
• ADAMTS13 activity (if TTP suspected)
• Complement levels (if aHUS suspected)

Renal ultrasound to exclude obstruction and assess kidney structure 2

Kidney biopsy is rarely required and should be reserved for cases where diagnosis will change management, preferably before the third trimester 2, 4. The risks must be carefully weighed against benefits.

Common Pitfalls to Avoid

1. Physiologic creatinine decrease in pregnancy masks early AKI: Normal pregnancy reduces serum creatinine to 0.4-0.8 mg/dL. A "normal" creatinine of 1.0 mg/dL may represent significant kidney injury in a pregnant patient 2, 9

2. Delayed recognition of P-TMA: Assuming all postpartum AKI is HELLP and waiting for spontaneous improvement can be fatal. If no improvement occurs 48-72 hours post-delivery, immediately consider P-TMA and initiate plasma exchange 3

3. Overlooking nephrotoxic medications: NSAIDs, ACE inhibitors, ARBs are contraindicated in pregnancy and can precipitate AKI 10, 1

4. Missing septic abortion in first trimester: In regions with restricted abortion access, maintain high suspicion for septic abortion as a cause of early pregnancy AKI 3

Risk Factors for PrAKI

Identify high-risk patients for surveillance 1:

• Advanced maternal age
• Pre-existing comorbidities (diabetes, hypertension, chronic kidney disease)
• Multiple gestation
• Use of nephrotoxic medications
• Autoimmune disorders (especially SLE)
• History of preeclampsia

The incidence of obstetric AKI has paradoxically increased in developed nations over the past decade due to advanced maternal age and increasing comorbidities, despite declining in developing countries due to reduced sepsis-related causes 2, 4. This trend emphasizes the importance of preconception counseling and early risk stratification.