Lower Back Pain - Red Flags

In the medical word, the term "red flag" means a clinical indicator, a sign or symptom, that warrants further investigation. The intent is not to miss a serious underlying condition. The term was first used in the 1980's in the evaluation of lower back pain patients (Ref). There are red flags that are general and have many possible causes (e.g. unexplained weight loss) or it may be more specific (e.g. lower back pain with loss of bladder function).
When evaluating lower back pain, red flags (Ref) (Ref) (Ref) include
Recent trauma, especially in people older than age 50
Previous history of cancer
Recent bacterial infection
Loss of bladder function
Loss of bowel function
Pain that awakens you at night
Pain that lasts more than 6 weeks
Pain in persons younger than 18 years
Pain in persons older than 50 years
Unexplained weight loss
Fever (persistent) and chills
Night sweats
History of intravenous drug use
Lower back surgery within the last year
Saddle anesthesia (loss of sensation in your buttocks)
Prolonged use of corticosteriods
Pain that radiates below the knee
A significant loss of lower extremity strength
Any one of these red flags requires a careful examination to rule out potentially serious conditions.
References:
Red Flags for Low Back Pain Are Not Always Really Red
Evaluation of red flags minimizes missing serious diseases in primary care
Acute Low Back Pain: Recognizing the "Red Flags" in the Work-up