Several blood tests are used to diagnose a lung infection, and the specific ones your doctor orders depend on how sick you are. The most common starting point is a complete blood count (CBC), which shows whether your immune system is actively fighting an infection. From there, tests like C-reactive protein (CRP), procalcitonin, blood cultures, and arterial blood gases may be added to identify the cause, gauge severity, and guide treatment.
Complete Blood Count (CBC)
A CBC is almost always the first blood test ordered when a lung infection is suspected. It measures the number and types of white blood cells circulating in your blood. White blood cells are your body’s infection fighters, so an elevated count signals that your immune system has been activated. A normal white blood cell count falls roughly between 4,000 and 11,000 cells per microliter of blood. In bacterial pneumonia, that number often climbs well above the upper limit.
The breakdown of white blood cell types also matters. A spike in neutrophils, the white blood cells that respond first to bacteria, points toward a bacterial cause. A rise in lymphocytes, on the other hand, leans more toward a viral infection. A white blood cell count that drops below 4,000 can actually be a warning sign of severe infection, and guidelines recommend additional testing, including blood cultures, when counts fall that low.
C-Reactive Protein (CRP)
CRP is a protein your liver produces in response to inflammation anywhere in the body. It’s not specific to lung infections, but it helps confirm that significant inflammation is present and gives a rough sense of severity. Normal CRP is typically below 10 mg/L. In one study of over 850 pneumonia patients, those with severe pneumonia were significantly more likely to have CRP levels at or above 10 mg/L compared to those with milder cases. Persistently elevated CRP can also signal that an infection isn’t responding well to treatment, so doctors sometimes recheck it over the course of your illness.
Procalcitonin: Bacterial or Viral?
One of the trickiest decisions in treating a lung infection is figuring out whether bacteria or a virus is responsible, because antibiotics only work against bacteria. Procalcitonin is a blood marker that rises sharply in bacterial infections but stays relatively low during viral ones, making it one of the most useful tools for this distinction.
In critically ill patients studied during the H1N1 influenza pandemic, those with bacterial pneumonia had a median procalcitonin level of 6.2 micrograms per liter, while those with purely viral influenza pneumonia had a median of just 0.56. Using a cutoff of 0.5 micrograms per liter, procalcitonin correctly identified about 80% of bacterial pneumonia cases. That accuracy improved to roughly 86% when patients with hospital-acquired infections or weakened immune systems were excluded. A low procalcitonin result can give your doctor confidence to hold off on antibiotics or stop them sooner, reducing unnecessary antibiotic use.
Blood Cultures
A blood culture checks whether bacteria from your lung infection have spread into your bloodstream, a serious complication called bacteremia. For this test, blood is drawn (usually from a vein in your arm) and placed in a container that encourages any bacteria present to grow so they can be identified.
Blood cultures are not routine for every case of pneumonia. They’re most useful in severe infections. Current guidelines recommend them when you’re sick enough to need intensive care, when imaging shows cavities in the lungs or fluid around them, when your white blood cell count drops below 4,000, or when you have chronic liver disease or alcohol use disorder. For mild to moderate community-acquired pneumonia, blood cultures rarely change the treatment plan.
The downside of blood cultures is speed. They have the longest turnaround time of common pneumonia tests, with a median of about 136 hours (roughly five and a half days) from sample to final result. Doctors start treatment before results come back, then adjust if the culture identifies a specific bacterium or reveals antibiotic resistance.
PCR Testing
Polymerase chain reaction (PCR) tests detect the genetic material of specific germs in your blood or sputum. They’re particularly valuable for identifying viral causes of pneumonia, like influenza or RSV, and for picking up bacteria that are difficult to grow in a standard culture. PCR results come back much faster than cultures, often within hours, which helps doctors narrow down the cause early.
Newer point-of-care tests can simultaneously measure two biomarkers from a simple finger prick: one protein that rises during viral infections and CRP, which rises during bacterial ones. These rapid tests deliver results in about 10 minutes and can help distinguish bacterial from viral infections right in the clinic, potentially avoiding unnecessary antibiotics or delays in the right treatment.
Arterial Blood Gas (ABG)
An arterial blood gas test isn’t used to diagnose the infection itself. It measures how well your lungs are doing their job of moving oxygen into your blood and carbon dioxide out. The sample is drawn from an artery, usually at the wrist, which makes it more uncomfortable than a standard blood draw.
Normal blood oxygen levels on an ABG fall between 75 and 100 mm Hg, and normal carbon dioxide levels sit between 35 and 45 mm Hg. Blood pH should be between 7.35 and 7.45. When pneumonia is severe, oxygen levels drop and carbon dioxide can build up, pushing the blood toward an acidic state called respiratory acidosis. This test is typically reserved for people who are visibly struggling to breathe or who may need supplemental oxygen or a ventilator. It’s worth noting that a normal oxygen reading doesn’t completely rule out respiratory trouble, especially if you’re already receiving supplemental oxygen. Carbon dioxide levels are actually a more sensitive indicator of how well your breathing is holding up.
How Long Results Take
Basic blood tests like a CBC and CRP are processed quickly, often within a few hours in a hospital setting. Procalcitonin results similarly come back the same day. Blood cultures are the slowest: the median time to a first microbiology result in hospitalized pneumonia patients is about 26 hours, but blood cultures specifically take a median of nearly 136 hours to finalize. The fastest results come from rapid antigen tests (around 3 hours) and PCR-based tests.
Because of these timelines, your doctor will typically start treatment based on your symptoms, chest imaging, and the fast-returning tests like CBC and CRP. Slower tests like blood cultures serve as a safety net, confirming the diagnosis and allowing treatment to be fine-tuned once results arrive.
Tests That Track Complications
When a lung infection is severe, it can strain organs beyond the lungs. Doctors may order a metabolic panel to monitor kidney and liver function, since markers of impaired kidney function are among the strongest predictors of a severe course. People with existing conditions like diabetes, liver disease, or chronic inflammation face higher risk of complications from pneumonia, so their bloodwork may be checked more frequently throughout treatment to catch early signs of organ stress.