How to Interpret ASO Lab Test Results

Bacteria that cause "strep throat," a common cause of a sore throat, are classified by laboratory analysis as "group A beta-hemolytic streptococci," or GABHS. A small percentage of patients with untreated strep throat recover from sore throat symptoms but then develop serious additional GABHS infection, causing rheumatic fever or kidney inflammation. Signs of rheumatic fever include fever, joint swelling, skin rash and heart conditions. Signs of kidney inflammation include bloody urine in addition to those of rheumatic fever.



Diagnostic tests for GABHS-related rheumatic fever or kidney inflammation include testing for blood antibodies--disease-fighting proteins produced by the body's immune system--against GABHS. The level of a specific antibody present in blood, reported as a "titer," reflects immune response to a specific infection. One of the antibody tests used for diagnosis of GABHS infection is the antistreptolysin O test, or "ASO" test.

Instructions

• 1

  Consult with a physician if signs of rheumatic fever or kidney inflammation occur. Report any history of sore throat. If the physician orders an ASO test, a blood sample will be taken and submitted to a laboratory. Often a second blood sample will be taken two weeks or more after the first sample.

• 2

  Acquire ASO test results from your physician or laboratory. Compare the ASO titer with the laboratory-reported classification of ASO titers. Compare the first titer to the second, or convalescent, titer. Variation in test results may occur among different laboratories, so use test guidelines from the specific laboratory used.

• 3

  Interpret test results with physician guidance. Laboratories may report titer levels as a number of international units per milliliter of blood (IU/ml) or as a ratio (for example, 1:2 or 1:4). Either an increased number of IU/ml or an increase in the second number of the ratio indicate an increased ASO titer.

  In general, the ASO titer rises until it reaches its highest level, about four weeks after the throat infection, then declines if the infection is resolving. A high initial ASO titer or an initial low titer followed by a significantly higher titer indicates recent or current GABHS infection. An initial low or negative titer followed by a second low or negative titer indicates no GABHS infection occurred, while a high titer followed by a low titer indicates a resolving or eliminated GABHS infection.

• 4

  Use ASO testing as an aid in testing for GABHS as a cause signs of rheumatic fever or kidney failure, when throat testing was not done during a sore throat which preceded these signs. Clinicians may order additional testing to determine whether GABHS is involved in kidney inflammation, because ASO testing is not accurate in detecting this disease in some cases.