Tests and Diagnosis
Diagnostic Injections — Searching for the Pain Source
When imaging doesn't clearly identify the cause, diagnostic nerve blocks may be used. A local anesthetic is injected near a suspected pain source to see whether the pain temporarily disappears.
Facet Joint Blocks (Medial Branch Blocks)
- Used to determine whether the facet joints (small joints at the back of the spine) are the pain source
- A single block has a false-positive rate of 27–63% (meaning it may incorrectly suggest the joint is the cause)
- For this reason, two separate blocks (using different anesthetics) are recommended (Cohen 2020)
- If both blocks provide relief, radiofrequency ablation (RFA) may be offered as a treatment
Sacroiliac Joint Blocks
- Used to determine whether the sacroiliac joint (in the pelvis) is the pain source
- Physical examination alone is unreliable for this diagnosis, so combining three or more clinical tests is recommended
Limitations of Diagnostic Injections
| Injection Type | Limitation |
|---|---|
| Facet joint block | High false-positive rate with a single block; "pure" facet joint pain accounts for only about 15% of cases |
| Sacroiliac joint block | Pain referral patterns are variable, making diagnosis difficult |
| Discography | Even pain-free individuals may report pain during the test |
| Selective nerve root block | Useful for identifying which nerve is responsible when multiple levels are affected |
Diagnostic injections are a useful tool, but they do not provide 100% certainty. Results should always be interpreted alongside your full clinical picture.