“Probiotic deficiency” is not a formal medical diagnosis because probiotics are not like vitamins/minerals with a straightforward blood marker. What people often mean is an imbalance in the gut microbiota (dysbiosis) and related symptoms. These can include prolonged diarrhea after antibiotics, recurrent gas/bloating, a marked change in bowel habits, and—in some people—broader issues such as skin complaints or increased susceptibility to infections. But these symptoms have many possible causes, so labeling them as “probiotic deficiency” can be misleading. If symptoms persist, consider diet quality (low fiber, high ultra-processed foods), sleep/stress, medications (especially antibiotics or PPIs), and conditions such as thyroid disorders or iron/B12 deficiency. Commercial stool microbiome tests exist, but they are not always decisive for clinical decisions. The most practical approach is symptom-focused medical evaluation plus nutrition optimization; if appropriate, a time-limited trial of an evidence-aligned probiotic/postbiotic matched to the goal.