Terrible pain left side of back under back ribs that radiates to the front but mostly the back & left side. could it be pancreatitis or spleen issues routine blood work 2 wks ago clear. how to tell the difference between muscular & serious issue?

Symptoms from pancreas or stomach is often associated with nausea & vomit. Muscular problem is usually precipitated or exacerbated with movement, somewhat relieved with local massage. Nerve pain, such as shingles-related neuralgia or post-herpetic neuralgia can radiate from back to front but always ipsilaterally. Left kidney pain can't be ruled out but it usually radiate to the groin + voiding sx.

Physical exam and detailed history usually guide in further selection of imaging tests that may or may not be diagnostic. Majority of cases have a musculoskeletal cause, visceral causes are ruled out if suggested by physical exam and history.