Hi  – great question. There are definitely clues that you can consider when looking at someone’s labs but you always have to consider the individual’s symptoms because labs are just 1 moment in time and impacted by many variables. In fact, I usually rely more on a person’s health history and symptoms, and reserve lab testing to confirm/rule out possibilities.

Allow me to share a few different scenarios to hopefully help answer your question:

When I would consider recommending digestive bitters:
Since digestive bitters stimulate all digestive secretions, I usually tend to recommend this over digestive enzymes when there’s a greater need/signs for bile support and stomach acid support.

Symptoms: 
• if someone has signs of low bile production/flow: i.e. constipation, slow motility/gastroparesis, SIBO/SIFO, gas/bloating, fatty stools (if stool floats, is greasy, sticky), nausea, easily gets an upset stomach especially with fatty foods, easily gets drunk with a little alcohol, sensitive to chemical fumes.
• if someone has signs of low stomach acid: i.e. belching/burping and bloating shortly after a meal, heartburn/reflux, stomach pains, confirmed H Pylori overgrowth, being vegan or loss of taste for meat, prefers not to eat meat or prefers to skip meals, bad breath, maldigested food in stool.

Labs: 
• low fat soluble vitamins (A,D,E,K), direct bilirubin >0.2 (can show biliary tract congestion), low nutrients (calcium, iron/ferritin, magnesium, zinc, B12), low Total Protein, high globulin, low chloride, positive H Pylori overgrowth.

When I would consider giving digestive enzymes:
This is a little more clear cut when it comes to labs because as you said, there is a marker that can be measured called Elastase-1. This measures total pancreatic enzyme output. It’s usually low in those with diabetes.

In a stool test, I like to see it above 500ug/g. However, keep in mind that lower elastase output can also be due to hypochlorhydria (low stomach acid) so this is where symptoms once again can help guide the recommendation for enzymes or bitters or both.

Symptoms:
• If someone clearly has trouble breaking down certain food groups like carbs or proteins or fats which causes bloating, gas, maldigestion, seeing undigested food in stool, etc., they may benefit from a targeted digestive enzyme. 
• In cases of traumatic life events or gastric surgery or diabetes, enzymes can be helpful.

Hope this helps answer your question.