What tests confirm Sjögren’s and kidney involvement specifically?

The short version: no single test confirms both Sjögren's and kidney involvement at once. You need separate tests for each, and the kidney tests are the ones that matter most for your health.

For Sjögren's itself, the 2016 ACR-EULAR criteria give the most specific diagnosis. They combine a labial salivary gland biopsy showing a focus score of 1 or higher (weight 3 points) with a positive anti-SSA/Ro antibody test (weight 3 points), plus eye and mouth dryness tests that add smaller weights [1]. A total of 4 or more points out of 9 classifies you as having Sjögren's. The biopsy alone has about 95% specificity when combined with anti-SSA/Ro testing [5], but it is an invasive procedure and the focus score can vary between samples [2]. The anti-SSA/Ro blood test is the most practical starting point.

For kidney involvement, the evidence here is much thinner. The most specific test is a kidney biopsy. One case series of 25 primary Sjögren's patients who had kidney biopsies found a range of patterns: membranoproliferative glomerulonephritis with cryoglobulinemia, immune-complex glomerulonephritis, IgA nephropathy, and tubulointerstitial nephritis [22]. Another case report showed cryoglobulinemic glomerulonephritis with vasculitis, diagnosed only by biopsy [21]. The biopsy is the gold standard because blood tests and urine tests alone cannot tell you which pattern of kidney damage you have, and the pattern determines treatment.

The non-invasive clues are urine protein, urine blood, serum creatinine, and cryoglobulins. In the biopsy series, every patient with kidney involvement had either proteinuria or hematuria on urinalysis [22]. The cryoglobulin case had a urine protein-to-creatinine ratio of 8,790 mg/g and a creatinine that peaked at an eGFR of 5 mL/min [21]. But these are screening tests, not confirmatory ones. A normal urinalysis makes significant kidney involvement unlikely, but an abnormal one does not tell you the cause.

My call: start with anti-SSA/Ro and a urinalysis with microscopy. If the urinalysis shows protein or blood, get a kidney biopsy. The salivary gland biopsy is for confirming Sjögren's when the blood test is negative or the diagnosis is uncertain. Confidence: moderate. The evidence supports the biopsy as the gold standard for kidney involvement, but the sample sizes are small and no study here directly compares biopsy results to non-invasive tests in a large Sjögren's population.