Rickettsia rickettsii and Coxiella burnetii (Q Fever) Antibody Panel, Phase 1 and Phase 2 Blood Test

What is this test? This is a blood test that looks for antibodies your immune system makes against two bacteria: Rickettsia rickettsii, which causes Rocky Mountain spotted fever, and Coxiella burnetii, which causes Q fever. Your provider may order it if you have fever, headache, rash, muscle aches, or fatigue after possible exposure to ticks or to cattle, sheep, or goats.

This test is a blood test that looks for IgG antibodies in your blood. IgG antibodies are proteins your immune system makes to fight infections. The test checks for antibodies against two different bacteria: Rickettsia rickettsii, which causes Rocky Mountain spotted fever, and Coxiella burnetii, which causes Q fever.

What are Rickettsia rickettsii and Coxiella burnetii? Rickettsia rickettsii is a bacterium transmitted mainly through the bite of infected ticks and causes Rocky Mountain spotted fever, a serious illness marked by fever, headache, and a characteristic rash. Coxiella burnetii is a hardy bacterium that causes Q fever; people most often catch it by inhaling contaminated dust from birthing fluids, manure, or unpasteurized dairy products of infected farm animals.

Rickettsia rickettsii is a bacterium spread by tick bites that causes Rocky Mountain spotted fever. This illness can cause fever, headache, rash, and can affect many organs including the heart. Coxiella burnetii is a bacterium that causes Q fever, which people can get from breathing in dust contaminated with animal birth fluids or from drinking unpasteurized milk. Q fever can affect the liver, lungs, and heart.

What do phase 1 and phase 2 mean for Coxiella burnetii? Coxiella burnetii exists in two antigenic forms known as phase 1 and phase 2. Antibodies against phase 2 antigens usually rise early during acute Q fever, while high or persistent levels of phase 1 antibodies often point to chronic, long-lasting infection. Measuring both phases helps your provider distinguish a recent infection from a chronic one.

Coxiella burnetii has two different forms called phase 1 and phase 2. Phase 1 is associated with acute (recent) infection, while phase 2 is associated with chronic (long-lasting) infection, particularly of the heart valves. Testing for both phases helps your doctor understand if your infection is recent or long-standing.

How is the test performed? A healthcare professional cleans your skin and draws a small blood sample from a vein, usually on the inside of your elbow. The sample is sent to a laboratory, where testing detects and measures antibodies against each bacterium. Because antibody levels can rise over time, a second sample drawn two to four weeks later may be requested to confirm the result.

A healthcare professional will draw a small amount of blood from a vein in your arm. The blood sample is sent to a laboratory where it is tested to see if it contains IgG antibodies against these bacteria. You may feel a brief sting when the needle is inserted, and there may be slight bruising afterward, but serious complications are rare.

What do the results mean? A negative result usually means there is no current or past infection, though very recent infection may not yet produce detectable antibodies. A positive or rising phase 2 antibody level suggests recent or acute Q fever, while high phase 1 antibodies can indicate chronic infection. Antibodies to Rickettsia rickettsii suggest exposure to the bacteria that cause Rocky Mountain spotted fever. Your provider will interpret these results together with your symptoms, travel and exposure history, and physical exam, since past exposure can also cause positive findings.

A positive result means your body has made IgG antibodies against these bacteria, suggesting you have been exposed to Rickettsia rickettsii or Coxiella burnetii. Your doctor will interpret the results along with your symptoms, medical history, and other test results to determine if you have an active infection or had a past infection. A negative result means no antibodies were detected, but this could also mean the infection is very recent and antibodies have not yet developed.