From the Guidelines
The most appropriate next step in diagnosis for this 3-year-old girl is measurement of serum C3 concentration (option A). 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. The decreased serum C3 level is characteristic of this condition, as the immune complex deposition activates the complement cascade, leading to complement consumption. This test will help confirm the diagnosis of post-streptococcal glomerulonephritis, which is the most likely diagnosis given her presentation with edema, hypertension, and urinary abnormalities occurring approximately 10 days after a throat infection. The elevated BUN and creatinine also indicate acute kidney injury, which is consistent with this diagnosis. Measuring C3 levels is more specific and appropriate than the other options for this particular clinical scenario, as supported by the KDIGO 2021 guideline for the management of glomerular diseases 1.
According to the guideline, the evaluation of classic bacterial infection-related GN syndromes, such as post-streptococcal glomerulonephritis, should include assessment for low complement (C3, C4) levels 1. The ACR appropriateness criteria for hematuria in children also recommend a thorough evaluation of the urine, including assessment for proteinuria, red blood cell casts, and deformed red blood cells, which are all present in this case 1.
The other options, such as measurement of serum IgA concentration (option B), serum antiglobulin assay (option C), and serum antinuclear antibody assay (option D), may be relevant in other clinical scenarios, but are not the most appropriate next step in diagnosis for this patient. Urine protein electrophoresis (option E) is also not the most relevant test in this case, as the primary concern is the diagnosis of post-streptococcal glomerulonephritis, rather than a urinary tract infection or other conditions that may cause proteinuria.
In summary, the clinical presentation and laboratory findings in this case are consistent with post-streptococcal glomerulonephritis, and measurement of serum C3 concentration is the most appropriate next step in diagnosis. This is supported by the KDIGO 2021 guideline and the ACR appropriateness criteria for hematuria in children 1.
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 findings, including low serum C3 concentration, hematuria, and proteinuria, are consistent with APSGN 2, 3, 4. The presence of RBC casts in the urine also supports this diagnosis.
Next Step in Diagnosis
Given the patient's symptoms and laboratory findings, the most appropriate next step in diagnosis is to measure the serum C3 concentration 2, 3, 4. This test can help confirm the diagnosis of APSGN and distinguish it from other causes of glomerulonephritis.
Key Points
- APSGN is an immune-complex mediated glomerular disease triggered by group A β-hemolytic streptococcus (GAS) or Streptococcus pyogenes infections 4.
- The disease is characterized by hematuria, oliguria, hypertension, and edema 2, 3, 4.
- The prognosis for APSGN is generally good, but some cases may evolve into chronic kidney disease 4.
- Measurement of serum C3 concentration is an important diagnostic test for APSGN 2, 3, 4.
Possible answers:
- A) Measurement of serum C3 concentration
- The other options are not supported by the evidence:
- B) Measurement of serum IgA concentration: not relevant to the diagnosis of APSGN
- C) Serum antiglobulin assay: not a standard diagnostic test for APSGN
- D) Serum antinuclear antibody assay: not relevant to the diagnosis of APSGN
- E) Urine protein electrophoresis: not a standard diagnostic test for APSGN