TAS: antistreptolysin titre: examination, values, what it is for
The TAS is the antistreptolysin test, also called ASO or ASLOT, is a blood test that checks for the presence of an infection by the Streptococcus bacterium. When we come into contact with harmful germs, our body produces specific antibodies for each pathogen it has to defend against.
The antistreptolysin titre test (TAS) measures the antibodies produced by the immune system in response to a toxin known as Streptolysin O, produced by group A Streptococcus (GAS) bacteria.
TAS: what is it and what does it measure?
ASLOTs begin to grow approximately 14 days after Streptococcus infection, reaching a maximum level at 3-5 weeks, and slowly return to normal over approximately 6-12 months.
They are also present in small amounts in the blood of healthy people (150-400 IU/ml) following previous infections with streptolysin-producing streptococci, most of them minor.
However, an increase in ASO does not equate to a diagnosis of acute rheumatoid arthritis. Indeed, without an inflammatory joint context it is the sign of a previous infection, a normal response of the body and does not require antibiotic treatment.
The values vary with age, seasonality and geographical factors that influence the frequency and intensity of streptococcal disease.
Some studies have shown that only 0.5-3% of patients who have had strep throat later develop acute rheumatoid arthritis, along with a number of genetic factors that contribute to it.
Fortunately, not all streptococcal pharyngitis gives systemic complications. The clinical picture of acute rheumatoid arthritis is polymorphic and differs in children from adults.
In the context of rheumatic disease, the antistreptolysin titre (ASO) reaches very high values (1000-1200 IU/mL).
Why is the TAS performed?
Doctors recommend TAS or ASLOT if you have symptoms of post-streptococcal complications. Some of the more common include:
- bacterial endocarditis.
- Glomerulonephritis.
- Rheumatic fever (also called active rheumatic arthritis).
Antistreptolysin antibodies reach their highest level in the system three to five weeks after a strep infection.
Levels can remain high for several months. Your doctor can determine if some symptoms are due to a post-streptococcal complication by checking the level of antibodies.
If you are interested in the topic, discover our study on streptococcus.
Streptococcus infections
Streptococcus infections are extremely common in the population, against them the body manifests an immune defense reaction by secreting specific antibodies against the microbial antigen.
Of all the streptococci, only group A haemolytic streptococcus is truly dangerous, due to the complications it can induce.
Acute rheumatoid arthritis (also called rheumatic fever), for example, is a disease that has as its etiology the previous infection with haemolytic streptococcus A.
It can affect various tissues, mainly the heart and joints, but sometimes also the central nervous system and skin.
Group A streptococci secrete over 20 toxins and enzymes, the most important of which are:
- streptolysin O and S: stimulates the production of TAS antibodies.
- Hyaluronidase: stimulates anti-hyaluronidase.
- Streptochinasi: genera l’anti-streptochinasi.
- Diphosphopyridine nucleotidation.
- Deoxyribonuclease: Stimulates anti-DNAse-B.
- Proteinasi streptococcica.
- Erythrogenic toxin.
Streptolysin O has been shown to be lethal when injected intravenously into experimental animals and causes a strong immune response in the body if the infection is left untreated. The antibodies produced by the body against streptolysin O are known as ASOs.
The most common strep infections are in the upper respiratory tract: angina (throat), nasopharyngitis, scarlet fever, or on the skin, although skin infections do not cause acute rheumatoid disease.
The contagious state is maximum in the initial periods of the disease, with a short incubation of about 48 hours. In the absence of specific antibiotic treatment, streptococci persist in the airways for weeks or months, with patients becoming carriers and contributing to the spread of germs even if the signs of pharyngitis have disappeared.
The level of antibodies to streptolysin O, ASO, is determined in the blood of patients and allows for the identification of infection or carrier status.
Antistreptolysin: what it is and how it acts in response to Streptococcus
Group A beta-hemolytic streptococci produce a variety of toxins that can act as antigens.
One of these exotoxins is Streptolysin O. Anti-streptolysin O antibodies allow assessing the degree of infection with beta-hemolytic streptococci. The ASO level increases one week after infection, peaking after 3-5 weeks.
In the absence of complications or reinfections, the value returns to normal after 6-12 months.
Preparation for the TAS exam
Your doctor will tell you if you need any special preparation for the test. For example, you may not need to eat or drink anything up to six hours before the test.
Another recommendation might be to stop taking some medicines such as corticosteroids or some antibiotics, as these can reduce the levels of ASO antibodies. This could make it difficult to confirm the diagnosis.
It is necessary to inform the specialist about all the medicines you are taking, including supplements and over the counter medicines.
In the same way, the intake of any medicine should not be interrupted, unless otherwise instructed by the doctor.
How is the TAS exam carried out?
The test is done by taking a sample of venous blood.
A nurse or lab technician will draw a small amount of blood from a vein on the inside of your arm or hand. A needle will be used to puncture the vessel and draw blood into a test tube.
Then the sample will be sent to a laboratory for analysis. The doctor will share his opinion with the patient once he has seen the report.
Are there any complementary exams to the TAS?
To detect the presence of an active infection, the doctor may also order a complete blood count, as well as an ESR and CRP test (especially in children and infants, very useful indicators of systemic inflammation).
Often the investigation can be integrated with the search for anti-streptokinase, anti-hyaluronidase and anti-DNase B antibodies, again by serological sampling.
Sometimes, in acute cases, the search for Streptococcus in the throat is suggested by performing an oropharyngeal swab.
The results arrive within 24-48 hours and are able to detect the presence of the bacteria in the throat.
TAS in blood: normal, high and low values
Normal values
In general, an ASO test value below 200 IU is considered normal.
In children under 5 years of age, the test value should be less than 100. However, results vary by laboratory.
High values
Elevated TAS levels (above 200 IU) indicate an acute or recent infection.
The ASO response, however, is not present in all cases, therefore, only 85% of patients with acute rheumatoid arthritis appear to exhibit elevated values.
Skin infections, unlike throat infections, are associated with a poor TAS response. Thus, patients with acute glomerulonephritis secondary to cutaneous streptococcal infections have an attenuated immune response to streptolysin O.
For these, an alternative test is recommended, such as anti-DNase B antibodies (antistreptodornase B, which has a higher sensitivity than TAS).
Low values
If the TAS result is negative but your doctor still thinks you may have a post-streptococcal complication, a second type of antibody test may be done for further investigation.
Your doctor may repeat the test within 10 to 14 days to accurately confirm the results. The body produces ASO antibodies within a week of the primary infection.
If both tests are negative, your symptoms aren’t due to a strep infection, although your doctor may recommend another antibody test to be sure.
If the results of the second test show antibodies are rising, the infection is probably recent. The decrease in the levels of the same indicates instead that the infection is improving.
TAS high: what to do?
Antibiotic treatment aims to eradicate the streptococcal infection and is given after a definitive diagnosis of infection, for example, strep throat.
In the case of acute reactive arthritis, treatment also includes an anti-inflammatory drug and specific therapies for cardiac and neurological complications.
The doctor will decide the indication for secondary prophylaxis after a streptococcal infection, i.e. the administration of an antibiotic to prevent recurrence of acute reactive arthritis (also called rheumatic fever).
The duration can vary, in the most difficult cases, from 5 years to the rest of life. Patients who have experienced carditis or valvular disease during acute arthritis require longer treatment regimens.
Additionally, these patients will require bacterial endocarditis prophylaxis in case of future dental extractions or surgeries in the eye-nose-throat, digestive, or genitourinary areas.
In particular, some studies suggest a prophylaxis scheme structured as follows, fever:
- rheumatic without carditis: prophylaxis for 5 years or up to 21 years of age.
- Rheumatic with carditis without results…
