Laboratory diagnosis of Rheumatic fever and Rheumatic heart disease

 

Preface

RF (Rheumatics fever) and RHD (Rheumatic heart disease) are non-suppurative complications of Group A streptococcal pharyngitis (sore throat) due to a delayed immune response typically affecting children of school-going age with a peak prevalence in the 5–14 years age group, and causing significant cardiac valve damage due to repeated episodes of ARF( Acute rheumatic fever).

In India, survey conducted in school children in the age group of 5-16 years by ICMR gives  prevalence of RHD as 6/1000(range 1.8 to 11/1000). RF is endemic in India and remains one of the major causes of cardiovascular disease, accounting for nearly 25-45% of the acquired heart disease.

 

Learning objectives

·        Pathogenesis of Rheumatic fever.

·        Clinical presentation in brief.

·        Microbiological tools for screening and diagnosis of RHD.

·        ASO (Anti Streptolysin O) test.

·        CRP (C Reactive Protein) detection test.

 

Pathogenesis of Rheumatic Fever

Autoimmune theory:

Streptococcal antigens show molecular mimicry with human antigens, hence antibodies produced against previous streptococcal infection (mostly sore throat) cross react with human tissue like heart, brain, joint, skin to produce lesions. Frequent episodes of Rheumatic fever can lead to cardiac lesions like inflammatory myocardial lesions, degeneration of heart valves which is known as RHD.

Clinical manifestations

Symptoms of rheumatic fever vary and typically begin 1 to 6 weeks after a bout of sore throat caused by group A streptococcus. Most common symptoms of RF maybe fever, swollen and painful joints, subcutaneous nodules, skin rashes, uncontrolled movements of arms, legs, or facial muscles etc. and those of RHD may be breathlessness, chest pain, swelling which may be accompanied by auscultatory murmurs.

Diagnosis

For diagnosis of RF, Jones criteria can be used which are in the form of major and minor criteria and supporting evidence of previous streptococcal infection.

Laboratory diagnosis

Laboratory tests like ASO titre as well as acute phase proteins detection like CRP, Procalcitonin, serum ferritin etc. play an important role in confirming a diagnosis and in the follow-up of rheumatic diseases.

Throat swab culture for presence of Group A Streptococcus helps in confirming one of the major Jone’s criteria for diagnosis of the disease.

ASO titre (Anti Streptolysin-O)

Antistreptolysin O (ASO) titre is a rapid latex agglutination test for the qualitative and semi-quantitative determination of antibodies against streptolysin O, a haemolytic exotoxin produced by group A Streptococcus bacteria.

Indications:

·        Rheumatic fever

·        Rheumatic heart disease.

·        Infective endocarditis due to Streptococcus

Procedure:

·        ASO latex reagent is a stabilized buffered suspension of polystyrene latex particles that have been coated with Streptolysin O.

·        Patient’s serum is added to this reagent following kit manufacturer’s guidelines and examined for agglutination.

Interpretation:

·        Most test have a detection limit of 200 IU/ml, i.e. positive agglutination indicates ASO level more than 200 IU/ml.

·        Rising titre is more significant in diagnosis of Acute Rheumatic Fever hence repeat test is recommended.

·        Antibiotics may give false negative results by inhibiting streptococcal antibody response, while increased Beta-lipoprotein levels, liver disease, and tuberculosis may give false positive results.

CRP (C Reactive Protein) detection test

 CRP is an acute phase reactant, a protein made by the liver and released into the blood within a few hours after tissue injury, the start of an infection, or other cause of inflammation.CRP was so named because it was first identified as a substance in the serum of patients with acute inflammation that reacted with the antibody against the somatic capsular polysaccharide (C-polysaccharide) of pneumococcus.

CRP detection and measurement techniques

·        Slide agglutination test (Qualitative and semi-quantitative)

·        Immunoturbidometry

·        Nephalometry

·        ELISA (Enzyme Linked Immunosorbent Assay)

·        CLIA (Chemiluminescent immunoassay)

Indications of CRP measurement test:

·        Bacterial infections including:

§  Meningitis and encephalitis

§  Infective endocarditis

§  Septicaemia

§  RHD

§  Pneumonia

·        Viral infections.

·        Rheumatoid arthritis.

·        Inflammatory bowel disease (Crohn’s disease, Ulcerative colitis).

·        Pelvic inflammatory diseases. (PID)

A high-sensitivity CRP (hs-CRP)

 Test measures low levels of CRP using laser Nephalometry.  The test gives results in 25 minutes with a sensitivity down to 0.04 mg/L.

The risk of developing cardiovascular disease is quantified as follows:

·         low: hs-CRP level under 1.0 mg/L

·         average: between 1.0 and 3.0 mg/L

·         high: above 3.0 mg/L

Reference range of S.CRP and clinical interpretation

·        Normal range: 0.3mg/dL- 0.8 mg/dL  ( 3mg/L – 8 mg/L )

·        Mild elevation up to 10 mg/L can be seen in obesity, pregnancy, depression, diabetes, common cold, gingivitis, sedentary lifestyle, cigarette smoking, etc.

·        False negative CRP can be found in patients with liver failure of clinical conditions having compromised liver functions.

·        Hepatic synthesis of CRP starts 6 to 8 hours after onset and peak concentrations are reached between 36 to 50 hours after infection has started.

·        The half-life of CRP is 19 hours making it come to normal range rapidly on treating the infection or inflammation, hence it has a significant value as a prognostic marker.

Proinflammatory cytokines (e.g., IL-1, IL-6, and tumour necrosis factor α-[TNF-α]) appear within one hour and Procalcitonin (PCT) after 5 hours of the start of bacterial infection.