What to Know About Antistreptolysin O Titer

Medically Reviewed by Robert Brennan on June 30, 2023
4 min read

An antistreptolysin O titer (ASO) is a blood test used to determine if you've had a recent infection caused by group A streptococcus bacteria. It's not used to diagnose a current strep infection, such as strep throat. Instead, this test is mainly used if you have symptoms that your doctor thinks may have been caused by a previous strep infection that went untreated. 

Bacteria called group A streptococcus (GAS) causes common infections such as strep throat and skin infections. GAS produces a toxic enzyme known as streptolysin O. Your immune system produces antibodies to fight this toxic enzyme. One of these antibodies is called antistreptolysin O (ASO).  

An antistreptolysin O titer measures how much ASO is in your blood. The ASO antibodies start increasing about a week after you experience a strep infection. They keep increasing for several weeks and then start to decrease. Health care providers typically don't use an ASO titer test to check for strep throat because the antibodies don't show up right away.

If you have a strep infection, it will most likely be treated with antibiotics and clear up. But if it doesn't cause you any symptoms or it's not treated completely, you may develop secondary complications. You may need an ASO titer blood test if your health care provider suspects you have one of the following complications.

Infections. Without proper treatment, strep bacteria can spread to other areas of your body. This can cause infections in your tonsils, sinuses, skin, middle ear, and blood.

Scarlet fever. This is most common in children between the ages of five and 15. Scarlet fever used to be considered a serious childhood illness. It can be treated with antibiotics now, but it can cause complications if it's not treated. Symptoms of scarlet fever include: 

  • Red rash that looks like a sunburn and feels like sandpaper
  • Red lines around the groin, armpits, elbows, knees, and neck
  • Flushed face with a pale ring around the mouth
  • A red and bumpy tongue that may be covered with a white coating early on

Rheumatic fever. This is an inflammatory immune response and not an infection. It's not contagious. Doctors believe it's your body's immune response to a previous strep or scarlet fever infection. Rheumatic fever is a serious condition that can affect your heart, joints, brain, and skin. The symptoms of rheumatic fever can include: 

  • Painful, tender joints at the knees, ankles, elbows, and wrists
  • Fever
  • Heart symptoms such as chest pain, shortness of breath, and a fast heartbeat
  • Tiredness
  • Uncontrollable jerking
  • Lumps under the skin near the joints, though this is rare
  • Rash of pink rings with a clear center, though this is also rare

Glomerulonephritis. This is a type of kidney disease. It damages the tiny filters inside your kidneys called glomeruli. It can cause your kidneys to have trouble removing waste and fluid from your body. Serious cases can cause kidney failure. You may not have any symptoms of glomerulonephritis. If you do have symptoms, they may include: 

This test uses a blood sample drawn from a vein, usually in your hand or arm. Your doctor may tell you not to eat for six hours before the test. Your doctor may do another ASO titer test two weeks later to see if your levels are increasing or decreasing.

A negative ASO level or a very low ASO level means that you probably haven't had a strep infection recently. Your body starts producing ASO antibodies between a week and a month after you're infected. Three to five weeks after you develop the illness, you will have the most antibodies in your blood. After that, they'll start to decrease. ASO antibodies will remain detectable in your blood for several months after an infection. 

If you have elevated levels of ASO antibodies, it means that you have probably had a recent strep infection. If your levels are taken again later and are higher, it also means you've probably had a recent strep infection. If your levels are taken again sometime after that and are decreasing, it means your infection may be clearing up.   

If your health care provider thinks you may have strep throat now, you might need a rapid streptococcal antigen test or a throat culture to check for GAS bacteria. If you have an infection in other parts of your body, you may need to have other cultures done of your blood, tissues, or mucus in your lungs.  

If your healthcare provider thinks you may have rheumatic fever, you might need to have other antibody tests done. Anti-DNAse B, anti-hyaluronidase, or anti-streptozyme are other antibodies that may be present in your blood. They can help your doctor determine if you have rheumatic fever.