Generality
The antistreptolysinic title is a blood test that allows to establish the presence in the body of a streptococcal infection .
This test is useful for measuring the amount of a particular type of antibody (called antistreptolysin ), specifically produced following contact with these bacteria. More precisely, the antistreptolisina serves to counteract the streptolysin, a protein with a haemolytic action produced by some streptococci (groups A, C and G).
If the antistreptolysinic titre is low or the test is negative it means that a streptococcal infection has not occurred. A high or increasing value can instead signal a recent exposure to the pathogen.
Streptococcus is mainly responsible for throat infections (pharyngitis, tonsillitis, etc.) and skin infections, which can lead to serious complications, such as acute glomerulonephritis, rheumatic fever and endocarditis.
Once the presence of the bacterium has been ascertained with the determination of the antistreptolysinic titre and other tests, it is possible to intervene with a targeted antibiotic therapy.
What's this
The antistreptolysinic title, or TAS, is a serological test that allows to quantify the concentration of antistreptolysin present in the bloodstream.
What is Antistreptolysin?
Antistreptolysin O (ASO, "Anti-Streptolysin O") is a special type of antibody that is produced by the immune system in response to streptococcal infection . More precisely, this is synthesized to neutralize the hemolytic properties of streptolysin O, a protein produced by most strains of group A streptococci and by many of those belonging to groups C and G. The letter "O" indicates that this immunogenic toxin is labile to oxygen.
Streptolysin O is one of the two hemolysins (the other is streptolysin S ) produced practically by all strains of Streptococcus pyogenes (or group A beta-hemolytic streptococcus). In addition to causing hemolysis of red blood cells, this toxin has direct toxic effects on heart tissue.
The antistreptolysinic title is also called with the abbreviation TAS (abbreviation of "Anti Streptolysinic Title") and the acronyms ASLO or ASLOT (from the English "AntiStreptoLysin O Titer").
Streptococcus: basic notions
Streptococcus is a widespread pathogen. This bacterium typically localizes at the level of the upper respiratory tract, where it is able to cause pharyngitis : when the organism is weakened, the streptococcus multiplies near the oral cavity, causing high fever (up to 39-40 ° C), chills, bad breath, swelling of the tonsils, difficulty swallowing and whitish plaques in the throat. Streptococcus is responsible for a variety of other infections, including skin infections such as pyoderma, impetigo and cellulite .
In most cases, streptococcal infections are identified and managed with antibiotic therapy until the disease is resolved.
However, if an infection does not present a clear symptomatology, it is not detected and eliminated with a targeted therapy, the bacterium remains latent in the body, infiltrating vital organs, such as the heart or kidneys . This can lead to post-streptococcal complications, which are especially dangerous in young children, such as glomerulonephritis (inflammation of the renal glomeruli), rheumatic fever or myocarditis . The secondary consequences to these pathologies are rather serious and include heart failure and renal dysfunction.
Why do you measure
The antistreptolysinic titre test measures the amount of antistreptolysin found in the blood stream.
This determination is useful to establish whether a patient has recently contracted a streptococcal infection (also called streptococcosis ). If the value is high, it means that the subject's contact with the bacterium has been recent or ongoing.
Furthermore, performing the TAS may help to clarify the clinical picture in the presence of easily recognizable symptoms, to determine whether a recent streptococcal infection has caused a glomerulonephritis or an episode of rheumatic fever.
TAS: after how long can you measure it?
In order to detect the presence of antibodies by examining the antistreptolysinic titre, it is necessary to wait at least a week after the streptococcal infection has been contracted. The antistreptolisina is not, in fact, immediately produced, but increases after 7 days from the infection, reaching the peak within 4-6 weeks, to then decrease the resolution of the infection and return to normal values after a period of time that can last even a year.
Consequently, the examination of the antistreptolysinic titre cannot be used for the diagnosis of acute infections. In these cases, however, a rapid test for the detection of streptococcus or a pharyngeal swab (culture test) can be performed.
When is the exam prescribed?
In general, the dosage of the antistreptolysinic titre is required a few weeks after the onset of symptoms indicative of a pharyngotonsillitis and / or a streptococcal skin infection.
Therefore, the TAS evaluation is indicated by the doctor when the patient experiences some of the following symptoms:
- Sore throat;
- Temperature;
- Chest pain;
- Fatigue;
- Palpitations;
- Breathing difficulty;
- Edema;
- Dark urine;
- Rash;
- Swelling or pain in the joints.
To understand if the antistreptolysinic titre has increased, decreased or remained stable, the doctor can also schedule the execution of this examination twice, within ten days of distance, in order to obtain the titles of ASO. This double check also makes it possible to distinguish a state of convalescence from an ongoing infection.
The antistreptolysinic titre test can also be prescribed if the doctor suspects that the patient has a disease resulting from a recent streptococcal infection, not previously identified and eradicated with a targeted therapy. The examination is then requested at the onset of symptoms indicative of rheumatic fever or glomerulonephritis, as a support to the diagnosis.
Related Exams
The antistreptolysinic titer is positive only in about 80% of streptococcal infections, so a negative test does not necessarily exclude the diagnosis (note: the TAS may be low or negative if the infection is caused by strains that are scarce producers of streptolysin O ). Similarly, undergoing the TAS test alone has proved to be of little use, since streptococcal infection may have already passed.
For this reason, the antistreptolysinic title is frequently prescribed together with the ESR, ie the determination of the erythrocyte sedimentation rate (inflammatory index). If both these parameters are positive, it means that streptococcal infection is ongoing. In this case, it is advisable to do other tests to assess what kind of treatment is needed to solve the problem.
The determination of the antistreptolysinic titer can be requested alone or in association with the anti-DNase B ( anti-deoxyribonuclease B ) test, another test used to assess the presence of a recent streptococcal infection. Measured above all when there is suspicion of a false negative (20% of the cases), this last parameter, if combined with the ASO title, can detect up to 95% of streptococcal infections.
Normal values
In adults, the normal reference value of the antistreptolysinic titer in the blood is less than 200 IU / ml.
- Values above 200 IU / ml: TAS positive .
- Values lower than 200 IU / ml: negative TAS .
In children, the antistreptolysinic titer is considered positive when it is higher than 350-400 IU / ml.
Note : the reference interval of the exam can change according to age, sex and instrumentation used in the analysis laboratory. For this reason, it is preferable to consult the ranges listed directly on the report. It should also be remembered that the results of the analyzes must be assessed as a whole by the general practitioner who knows the patient's medical history.
High Antistreptolysinic Title - Causes
When the antistreptolysinic titre test is positive it means that a streptococcal infection may be present.
More precisely, the possibilities are two:
- Streptococcal infection is still ongoing (acute infection);
- The infection has recently been contracted and eradicated (convalescence).
In general, a high level of TAS is associated with the following pathological conditions:
- Laryngitis ;
- Tonsillitis ;
- Pharyngitis ;
- Scarlet fever (exanthematous disease);
- Erythema nodoso (infection of the skin that causes eruption of reddened nodules).
A high value of the antistreptolysinic titer with respect to the reference one may also indicate the presence of more serious post-streptococcal diseases, complications of previous infections that are not well treated, such as:
- Endocarditis;
- Myocarditis ;
- Glomerulonephritis ;
- Rheumatic fever ;
- Toxic shock syndrome .
Post-streptococcal complications: a brief note
The introduction of rapid tests for the diagnosis of these infections has decreased the frequency of these complications, which are however still present. The dosage of the antistreptolysinic titer can be used to determine whether these pathological conditions are actually consequent to a recent streptococcal infection.
Low Antistreptolysinic Title - Causes
Low values of antistreptolysinic titre are not usually associated with medical problems and / or pathological consequences, therefore they are not considered clinically relevant. This means, in fact, that the body is not producing the antistreptolysin antibody and, consequently, the patient is not affected by any type of streptococcal infection.
How to measure it
A venous blood sample is taken from the arm to determine the antistreptolysinic titer. The dosage is performed with serological methods, such as latex agglutination test or slide agglutination (Widal test).
Furthermore, it is possible to perform tests such as the ELISA (Enzyme-Linked Immunosorbent Assay) to detect the exact value of the antistreptolysinic titer.
Preparation for the exam
Before the useful sampling for the determination of the antistreptolysinic titre, a particular preparation is not necessary, but, for safety, it is advisable to always stick to the advice that your doctor will provide.
As with other blood tests, even in the case of the TAS assessment, factors may influence the outcome of the exam . For example, taking some antibiotics and corticosteroids can decrease antibody levels.
False negative results of the antistreptolysinic titre may also result from liver disease and tuberculosis .
Interpretation of Results
Streptococcal infections must be diagnosed and treated promptly, in order to avoid complications. The antistreptolysinic titre is detectable after at least a week from exposure to the bacterium; the exam is therefore not useful in the diagnosis of an acute infection. In this regard, the doctor can use culture analysis on pharyngeal or cutaneous swab and the rapid test for streptococcus.
In this way, streptococcal infections can be identified and managed with appropriate antibiotics, until complete resolution of the disease.
Antistreptolysinic titre values
The negative outcome of the examination, or the presence of ASO in very low concentrations, indicates that a streptococcal infection probably did not occur. This is especially true if a sample taken after 10–14 days is still negative.
High values of antistreptolysinic titre
If the antistreptolysinic titer is high (positive) or increasing, then it is likely that the patient has had a recent streptococcal infection. The progressive decrease in antibody levels indicates, instead, the presence of a recent infection in the process of resolution.
