MKSAP Quiz: 2-week history of fatigue, poor appetite, arthralgia, and rash
A 19-year-old woman is evaluated for a 2-week history of fatigue, poor appetite, arthralgia of the hands and knees, and a rash, all of which appeared 1 day after a beach trip. After physical exam, lab studies, and kidney ultrasound, what is the most appropriate diagnostic test to perform next?
A 19-year-old woman is evaluated for a 2-week history of fatigue, poor appetite, arthralgia of the hands and knees, and a rash, all of which appeared 1 day after a trip to the beach. She has no other medical problems and takes no medications.
On physical examination, vital signs are normal. A malar rash characterized by pink-violet papules and plaques with sparing of the nasolabial folds is noted. The remainder of the examination, including joint examination, is normal.
Laboratory studies:
| Albumin | 3.1 g/dL (31 g/L) |
| C3 | 50 mg/dL (500 mg/L) |
| C4 | 9 mg/dL (90 mg/L) |
| Creatinine | 1.1 mg/dL (97.2 µmol/L) |
| Antinuclear antibodies | Titer, 1:160 |
| Urinalysis | 3+ blood; 3+ protein; many erythrocytes; occasional dysmorphic erythrocytes; rare erythrocyte casts |
Kidney ultrasound shows kidneys of normal size and echogenicity.
Which of the following is the most appropriate diagnostic test to perform next?
A. Erythrocyte sedimentation rate
B. Extractable nuclear antigen panel
C. Kidney biopsy
D. Skin biopsy
MKSAP Answer and Critique
The correct answer is C. Kidney biopsy. This content is available to MKSAP 19 subscribers as Question 45 in the Nephrology section. More information about MKSAP is available online.
Kidney biopsy (Option C) is the most appropriate diagnostic test to perform next. The 2019 European League Against Rheumatism/American College of Rheumatology classification criteria for systemic lupus erythematosus (SLE) include having antinuclear antibody (ANA) positivity at least once as obligatory entry criterion, followed by additive weighted criteria grouped into seven clinical and three immunologic domains. Patients with the entry criteria of a positive ANA, at least one clinical criterion, and having accumulated ≥10 points are classified as having SLE with a sensitivity of 96.1% and specificity of 93.4%. This patient has a positive ANA, acute cutaneous lupus erythematosus, and low serum complement levels (10 total points). The classic pattern of lupus nephritis is an immune complex–mediated glomerulonephritis with a varied pathology that includes six distinct classes of disease. The biopsy is crucial not only to establish the diagnosis but also to indicate which class of lupus nephritis she has, as the approach to treating lupus nephritis is guided by histologic class and the degree of kidney function impairment.
Erythrocyte sedimentation rate (ESR) (Option A) measurement can be checked serially alongside other clinical parameters to gauge disease activity. However, an ESR measurement would not help in determining the class of lupus nephritis needed to choose optimal therapy.
This patient may test positive for one or more antibodies on the extractable nuclear antigen (ENA) (Option B) panel (e.g., anti-Smith, anti-U1-RNP, anti-Ro/SSA, anti-La/SSB), but this antibody panel test is not required to make a diagnosis of SLE. In addition, the results of an ENA panel are not expected to influence treatment plans compared with those of a kidney biopsy.
The diagnosis of acute cutaneous lupus erythematosus is primarily clinical. A skin biopsy (Option D) is indicated when the clinical diagnosis is in doubt. In this case, the appearance of the classic malar rash in the context of positive SLE-associated immunologic findings make a skin biopsy unnecessary. Finally, a skin biopsy would not shed any light on the patient's kidney manifestations or guide therapy for either the patient's skin or kidney disease.
Key Point
- Kidney biopsy is the definitive diagnostic test for lupus nephritis and, based on histologic findings, is crucial in determining treatment.