Understanding RPR 1:1
An RPR result of 1:1 represents the lowest dilution tested and is considered non-reactive/negative for syphilis. 1
What the Result Means
RPR 1:1 is effectively a negative result - this titer represents undiluted serum (no dilution performed) and indicates no clinically significant antibody activity. 2
The notation "1:1" means the test was performed on undiluted serum, which is the starting point for RPR testing before any serial dilutions are made. 1
This result does NOT indicate active syphilis infection when both the RPR is 1:1 and treponemal tests are negative. 3
Clinical Significance
RPR titers of 1:1 or 1:2 are typically seen in:
- Biological false positive reactions (low-titer reactivity from conditions like autoimmune disease, pregnancy, or viral infections). 2
- Very early primary syphilis before antibodies have fully developed (though this is uncommon). 4
- Successfully treated syphilis where the patient is becoming seronegative. 3
At titers ≥1:8, false-positive results are extremely rare, meaning titers of 1:1 or 1:2 have much lower specificity for true syphilis infection. 3
Interpretation Based on Clinical Context
If Treponemal Test is Negative:
- No treatment for syphilis is indicated - this represents either a biological false positive or no infection. 2
- Consider investigating underlying conditions that may cause false positive RPR (autoimmune disorders, viral infections, pregnancy). 2
If Treponemal Test is Positive:
- This pattern suggests previous treated syphilis with declining antibody titers. 1
- Nontreponemal tests have poor sensitivity (30.7-56.9%) for detecting past infection compared to 100% sensitivity in active infection. 5, 1
Important Caveats
Rapid syphilis tests may miss infections with RPR titers of 1:2 or lower, with detection rates as low as 7.6-56.5% at these titers. 6
In one study, all specimens with RPR 1:1 that were negative on rapid treponemal testing but positive on reference standard testing had this very low titer. 4
RPR results can vary significantly between laboratories - up to 3-fold differences have been documented, which could affect clinical decision-making. 7
If clinical suspicion for very early primary syphilis remains high despite RPR 1:1, consider repeat testing in 2-4 weeks or direct detection methods (darkfield microscopy, biopsy). 2