The monospot test is meant to deliver simplistic results and therefore is either positive or negative.  Positive results indicate the presence of heterophile antibodies in a patient with symptomatology consistent with IM.  A negative result is intended to indicate the lack of heterophile antibodies and should lead clinicians to pursue other causes of the patient’s symptomatology. 

There are multiple interfering factors with the monospot test, leading to significantly decreased levels of sensitivity and to decreased specificity as well.

Regarding the sensitivity of the test, there are many factors that contribute to high rates of false negative testing.  The first involved is patient age.  It has been demonstrated that children four years old and younger have unacceptably high false negative findings when compared to EBV-specific antibody testing.  In one study, children under the age of 4 who showed evidence of disease through EBV-specific antibody testing had agglutination testing sensitivity rates between 27 to 76%.[3][7]

Although sensitivity rates have been reported anywhere between 70% and 90%, these numbers appear to be somewhat dependent on the time at which the blood sample is taken.  Heterophile antibodies are present in peak concentrations between 2 and 6 weeks of disease.[6]  If testing occurs too early in the disease course, it may lead to further decreased sensitivity of the monospot test as concentrations of heterophile antibodies must reach a threshold level in the serum to be detected by the agglutination process.  High rates of false negatives have been identified with patients tested before 1-2 weeks of symptomatic onset.  Sensitivity rates peak at about 6 weeks of symptoms which may necessitate repeat testing in patients who continue to have symptoms, despite negative test results during previous health care encounters.[6]

In addition to varying sensitivity rates of detecting heterophile positive IM, the monospot test is unable to identify cases of heterophile negative IM.  Approximately 90% of IM cases are caused by EBV.[9] In cases of heterophile negative IM, the lack of agglutination causing antibodies will lead to a negative monospot result, despite persistent symptoms.  Since EBV is not the cause, the specific causative agent must be identified, usually with serum testing.  

The specificity of the monospot test is considered to be quite reliable and has a specificity rate between 95 to 100%.[5] Despite high rates of specificity, there have been some case reports detailing false positives from other disease processes such as Cytomegalovirus, human immunodeficiency virus, lupus, lymphoma, rubella, and herpes simplex virus.  It is also noteworthy that low levels of heterophile antibodies may persist at low levels for 9 months to 1 year after resolution of symptoms.[10]

Complications from the testing process are very rare.  Because testing requires obtaining a blood sample, complications are usually related to pain and bruising at the site of needle insertion and on extremely rare occasions, infection.  In the case of an intravenous blood sample, thrombophlebitis may occur as may persistent bleeding at the site, though these are extremely uncommon.

It is important for patients to recognize the limitations of the monospot test, particularly its lower rates of sensitivity.  If a patient undergoes monospot testing, particularly early on in the disease course, it is crucial that they continue to follow with their health care provider, especially if symptoms persist.  If IM is under suspicion as the cause of symptoms and monospot test has been negative, then it is important to discuss the utility of EBV specific antibody testing. 

Infectious mononucleosis is a rather common disease that is seen primarily in patients ages 10 to 30 years old.  Its symptoms, which include sore throat, fever, fatigue, and lymphadenopathy overlap with many other common disease processes.  Although the disease is treated with supportive care only, diagnosis is vital to initiate prevention of complications and ineffective or potentially dangerous treatments may be avoided. 

The monospot test is a particularly common test for the rapid detection of IM.  Results are obtainable within 5 to 10 minutes, and the cost of the test is relatively inexpensive, thus making it a very common testing modality in clinics and urgent care settings.  However, because of the test’s limited sensitivity, the Center for Disease Control does not endorse this modality of testing for general use or confirmation of EBV.

It is incumbent upon the practitioner to understand the appropriate use of the monospot test and the limitations inherent within the test, particularly with regards to its sensitivity.  In patients between the ages of 10 and 30 years old who have a constellation of symptoms consistent with IM, a positive monospot test may help to help to clarify the diagnosis of IM further, though EBV specific antibody testing should be a strong consideration. 

In patients presenting with Infectious mononucleosis-like symptoms who have a negative monospot test, false negative test results should be considered, especially if patient testing occurred within the first 1 to 2 weeks of symptoms.  It is crucial for the health care provider to consider EBV specific antibody testing or close follow up and repeat testing, depending on what resources are available. 

As with any diagnostic testing modality, one must understand the limitations of the test and the appropriate population in which to initiate testing.  Although it is a rapid and inexpensive test, there are significant concerns and limitations, particularly with the sensitivity of the test.  It should be utilized as a diagnostic aid in patients older than 4 years of age who present with symptoms that are clinically consistent with IM.  However, EBV-specific antibody testing is the recommendation for actual confirmation of the diagnosis of IM, due to EBV.