ASLO (Antistreptolysin O) — guide

What is ASLO

ASLO (Antistreptolysin O) — these are antibodies produced by the body in response to a Group A streptococcal infection. When streptococcus causes tonsillitis or pharyngitis, the bacterium releases the toxin streptolysin O, and the immune system responds by producing ASLO antibodies. This marker reflects not the infection itself, but the body's immune response to it.

The ASLO titer typically begins to rise 1–3 weeks after infection, peaks within 3–5 weeks, and gradually declines over several months. It is measured in blood serum in the laboratory and the result is expressed in international units per millilitre (IU/mL).

Reference ranges

In most laboratories, an ASLO titer of up to 200 IU/mL is considered normal for adults. However, it is important to know that normal ranges depend on age, geographic region, and season. In children older than 10 years, the baseline level is often higher than in younger children. In winter and autumn, when streptococcal infections are more frequent, population levels naturally rise.

A single elevated ASLO titer is not a diagnosis in itself — what matters is the trend: whether the titer is rising or falling.

Why ASLO may be elevated

The most common causes of an elevated ASLO:

• Recent Group A streptococcal infection (tonsillitis, pharyngitis, scarlet fever)
• Acute rheumatic fever — a serious complication of streptococcal infection that can affect the heart, joints, and nervous system
• Post-streptococcal reactive arthritis — joint pain and swelling following a streptococcal infection
• Post-streptococcal glomerulonephritis — kidney damage following infection
• Chronic or recurrent streptococcal infections, such as recurrent tonsillitis

Important: an elevated ASLO indicates contact with streptococcus, but does not necessarily mean a complication has already developed. A doctor evaluates ASLO together with other tests and clinical symptoms.

Why ASLO may be low

A low or undetectable ASLO titer usually means there has been no recent streptococcal infection. This is a normal state and generally gives no cause for concern.

In rare cases where streptococcal infection is suspected but ASLO is low, a doctor may order additional tests — anti-DNase B, blood culture, or a rapid streptococcal test — as not all streptococcal infections cause a significant rise in ASLO.

What to do next

If ASLO is elevated, it is important to discuss with a doctor:

• Whether there was recently a sore throat, tonsillitis, or another infection
• Whether there is joint pain, skin rashes, or cardiac symptoms
• Whether the test should be repeated in 2–4 weeks to monitor the trend
• Whether additional tests are needed (CRP, ESR, cardiac echocardiography)

Do not self-treat with antibiotics — a doctor will decide whether treatment is needed and what it should be.

Monitoring over time

The trend in ASLO titer is more informative than a single measurement. A rising titer indicates an active immune response, while a falling titer indicates the infection is resolving. Repeating the test is recommended after 2–4 weeks.

If ASLO was elevated due to acute rheumatic fever, long-term monitoring may be required for several years, as the condition can recur after new streptococcal infections.

Questions for your doctor

1. Does my elevated ASLO titer indicate an active infection, or only past contact with streptococcus? 2. Should I repeat the ASLO test in a few weeks to monitor the trend? 3. Should I have a cardiac evaluation (echocardiography) if ASLO was elevated after tonsillitis? 4. What symptoms should prompt immediate medical attention — joint pain, skin rashes, shortness of breath?