Doctors use specific clinical criteria to decide if a pneumonia patient needs hospitalization, primarily relying on severity scores like the CURB-65 or assessing vital signs. Key factors include low oxygen saturation (typically below 92%), respiratory rate (over 30 breaths per minute), low blood pressure, and confusion. Age is a significant risk factor, with patients over 65 often considered for admission due to higher complication risks. Other reasons for admission include a high fever that persists, pre-existing comorbidities like heart or kidney disease, and signs of complications such as sepsis or pleural effusion. Essentially, if a patient's body is struggling to fight the infection on its own or their vital signs are unstable, inpatient care becomes necessary.