What is the most appropriate next diagnostic step for a 3-year-old girl presenting with progressive edema, recent sore throat, and laboratory findings indicating impaired renal function, including proteinuria and hematuria, with RBC casts?

From the Guidelines

The most appropriate next step in diagnosis for this 3-year-old girl is measurement of serum C3 concentration. This child presents with classic features of post-streptococcal glomerulonephritis, including recent sore throat followed by hypertension, periorbital edema, and urinary findings of hematuria, proteinuria, and RBC casts. Decreased serum C3 complement levels are characteristic of this condition, as the immune complexes formed after streptococcal infection activate the complement cascade, leading to complement consumption 1. This test will help confirm the diagnosis of post-streptococcal glomerulonephritis, which is the most likely diagnosis given the clinical presentation, timing after pharyngeal infection, and laboratory findings showing evidence of nephritis. The hypocomplementemia typically resolves within 6-8 weeks as the condition improves, making C3 measurement both diagnostically and prognostically valuable.

Some key points to consider in this diagnosis include:

• The child's recent history of sore throat, which is a common antecedent to post-streptococcal glomerulonephritis
• The presence of hypertension, periorbital edema, and urinary findings of hematuria, proteinuria, and RBC casts, which are all consistent with glomerulonephritis
• The importance of measuring serum C3 concentration to confirm the diagnosis and monitor the disease course, as recommended by the KDIGO 2021 guideline for the management of glomerular diseases 1
• The need to rule out other causes of nephritis, such as other infections or autoimmune disorders, although the clinical presentation and laboratory findings in this case are most consistent with post-streptococcal glomerulonephritis.

The evaluation of hematuria in children, as outlined in the ACR Appropriateness Criteria for hematuria in children, also supports the measurement of serum C3 concentration as a key diagnostic step in cases with suspected glomerulonephritis 1.

Overall, the measurement of serum C3 concentration is a critical step in the diagnosis and management of this patient, and it is the most appropriate next step in diagnosis given the clinical presentation and laboratory findings.

From the Research

Diagnosis of Acute Poststreptococcal Glomerulonephritis

The patient's symptoms, including progressive swelling of the face, hands, and feet, mild periorbital edema, and crackles at both lung bases, suggest a diagnosis of acute poststreptococcal glomerulonephritis (APSGN) 2, 3, 4. The patient's recent history of a sore throat, which resolved spontaneously, also supports this diagnosis.

Laboratory Findings

The patient's laboratory results show:

• Low serum C3 concentration, which is a characteristic finding in APSGN 2, 3
• Presence of RBC casts in the urine, indicating glomerular injury
• Elevated blood pressure, which is a common feature of APSGN

Next Step in Diagnosis

Based on the patient's symptoms and laboratory findings, the most appropriate next step in diagnosis is:

• Measurement of serum C3 concentration, which can help confirm the diagnosis of APSGN 2, 3 Other options, such as measurement of serum IgA concentration, serum antiglobulin assay, serum antinuclear antibody assay, and urine protein electrophoresis, are not directly relevant to the diagnosis of APSGN.

Key Findings

Key findings that support the diagnosis of APSGN include:

• Recent streptococcal infection, such as pharyngitis or skin infection
• Latency period of 1-2 weeks between streptococcal infection and onset of nephritis
• Hypocomplementemia (low C3 level)
• Presence of RBC casts in the urine
• Elevated blood pressure and edema

Prognosis and Treatment

The prognosis for APSGN is generally good, with most patients recovering fully from the disease 2, 3, 4. Treatment is mainly supportive, with focus on managing hypertension, edema, and renal function. Loop or thiazide diuretics may be used to manage hypertension and edema, while angiotensin-converting enzyme inhibitors or angiotensin receptor blockers may be used to control hypertension, but with caution due to the risk of hyperkalemia 2.