Rheumatic fever (RF) is a serious inflammatory condition that develops as a delayed consequence of an untreated or inadequately treated infection caused by Group A Streptococcus (GAS) bacteria, most commonly strep throat. This reaction is an autoimmune response where the body’s defenses mistakenly attack its own tissues, particularly in the heart, joints, skin, and brain. Since there is no single definitive laboratory test for RF, diagnosis relies on a comprehensive assessment that combines evidence of a preceding strep infection with a specific pattern of clinical signs and symptoms.
Identifying Evidence of Preceding Strep Infection
The first step in confirming a diagnosis of rheumatic fever is establishing laboratory evidence of a recent Group A Strep infection. Since RF symptoms typically appear two to four weeks after the initial strep infection, a throat culture or rapid strep test for an active infection is often negative by the time a patient presents with RF. The primary method to confirm past exposure is by measuring the level of antibodies the immune system produced in response to the bacteria.
Two main antibody tests are utilized: the Anti-Streptolysin O (ASO) titer and the Anti-DNase B test. The ASO titer measures antibodies against streptolysin O, peaking around three to six weeks after infection. The Anti-DNase B test measures antibodies against another streptococcal enzyme and remains elevated for a longer period, making it useful when the ASO titer is negative. Combining both tests significantly increases the sensitivity to detect a prior strep infection, reaching approximately 95%.
Assessing Clinical Signs and Symptoms
Testing for rheumatic fever involves a careful physical examination to identify characteristic clinical manifestations, categorized as “Major” or “Minor.” The most serious major manifestation is carditis, inflammation of the heart tissue that can lead to permanent damage of the heart valves. A doctor listens for new or changing heart murmurs, which indicate abnormal blood flow caused by valve damage.
Major signs also include polyarthritis, a painful inflammation that migrates between large joints (knees, ankles, elbows, and wrists). Sydenham chorea is a neurological manifestation presenting as involuntary, jerky movements of the face and limbs, often occurring months after the infection. Less common major manifestations include erythema marginatum (a distinctive non-itchy rash) and subcutaneous nodules (firm, painless bumps under the skin). Minor manifestations include a fever generally above 38.5°C and arthralgia, which is joint pain without objective signs of inflammation.
Laboratory and Imaging Confirmation
Beyond the antibody tests, laboratory and imaging studies are crucial for confirming the presence of systemic inflammation and assessing organ damage. To measure the body’s inflammatory response, doctors routinely order the Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP) tests. Both are non-specific markers that become significantly elevated in acute rheumatic fever, indicating widespread inflammation in the body. The CRP level rises and falls more quickly than the ESR, making it a valuable tool for monitoring the patient’s response to treatment.
A specialized imaging test, the Echocardiogram (Echo), is an indispensable part of the diagnostic workup for RF. This ultrasound allows physicians to visualize the structure and function of the heart valves and muscles, detecting carditis even without an audible heart murmur. The Echo can identify subclinical carditis, heart inflammation only detectable by imaging, which is now recognized as a Major criterion in certain diagnostic guidelines. An Electrocardiogram (ECG) checks the heart’s electrical activity and may reveal a prolonged PR interval, a minor criterion suggesting inflammation of the heart’s conduction system.
Applying the Diagnostic Criteria
The final step in testing for rheumatic fever is synthesizing all collected data using the modified Jones Criteria. This clinical guideline provides a structured method for establishing a definitive diagnosis, acknowledging that no single test is sufficient. To confirm a first episode of RF, the patient must have clear laboratory evidence of a preceding Group A Strep infection.
In addition to the strep evidence, the patient must meet a specific combination of clinical findings. This requirement is typically either two Major manifestations (e.g., carditis and polyarthritis) or one Major and two Minor manifestations (e.g., carditis combined with fever and elevated inflammatory markers). The criteria are carefully applied; for instance, a patient cannot count both polyarthritis (Major) and arthralgia (Minor) simultaneously. This comprehensive methodology ensures accuracy and also helps classify the severity of the disease, particularly by confirming the presence or absence of carditis, which guides long-term management and prevention strategies.