SPUR: when to suspect an immunodeficiency
SPUR is an initial guide to suspecting immunodeficiency (an immune system defect): severe, persistent (failing to respond to appropriate treatment), unusual or recurrent (more frequent than expected for age) infections.
Remember
Think of immunodeficiency when infections are severe, persistent, unusual or recurrent (more often than expected for age). SPUR raises suspicion; laboratory tests then confirm the type.
Mnemonic
SPUR
- S Severe
- P Persistent
- U Unusual
- R Recurrent
Each letter is applied in turn to the infections in the patient's history.
An immunodeficiency is a defect in the immune system that leaves a person unable to control infections normally, and it usually shows itself first as infections. Clinicians often use the mnemonic (a memory aid) SPUR as an initial guide to when it should be suspected. It looks at the pattern of infections in a patient's history.
- Severe: could the infection be life-threatening if untreated? A healthy immune system usually keeps infection under control, so an infection that proves unexpectedly dangerous makes a clinician ask whether the body's defences are failing.
- Persistent: does the infection not respond appropriately to conventional treatment?
- Unusual: is the site or the microbe unusual, for example a deep tissue infection or an opportunistic microbe (one that rarely harms people with healthy defences)?
- Recurrent: are infections more frequent than average for a person without immune problems? Judge this against what is normal for the person's age and exposure, for example at nursery or school.
Meeting these criteria is a reason to look further, not a diagnosis. Recurrent infection is also commonly caused by something other than immune failure, such as inadequate antibiotic treatment or resistant organisms, so those are considered before concluding that there is an immune defect.
If the history fits, the next step is laboratory investigation to confirm the type of immunodeficiency. First, review the past medical history for a cause of secondary immunodeficiency, which is acquired through an underlying disease or treatment. A primary immunodeficiency, by contrast, is present from birth and caused by a gene defect, yet many patients are diagnosed as adults.
Key facts
- Purpose
- SPUR is an initial guide that raises suspicion of immunodeficiency; it is not a diagnosis.
- Severe
- an unexpectedly dangerous infection (one that could be life-threatening if untreated) makes a clinician ask whether the body's defences are failing.
- Persistent
- an infection that does not respond appropriately to conventional treatment is a key warning feature.
- Unusual
- a deep tissue infection or an opportunistic microbe (one that rarely harms people with healthy defences) fits this letter.
- Persistent thrush
- oral thrush (candidiasis, a yeast infection) that does not clear with appropriate treatment is a persistent, opportunistic infection, a recognised pointer to impaired immunity.
- Recurrent
- frequency is judged against what is normal for the person's age and exposure, for example nursery or school.
- Not covered
- some patients with primary immunodeficiency might present with autoimmunity (the immune system attacking the body's own tissues), cancer or inflammation, which SPUR does not cover.
Exam traps
- Several mild colds a year in a child at nursery do not count as a warning sign on their own: young children who mix with others normally catch several respiratory infections each year, so recurrence only counts when it is more than expected for age and exposure.
- Urinary infections that recur but each clear fully after a short course of antibiotics are a weaker warning sign: they are missing the persistent feature, and their site and organisms are ordinary rather than unusual.
- A single severe infection with a common pathogen can occur in an otherwise healthy person and does not show the persistent, unusual or recurrent pattern; severity alone is less specific than the overall pattern of infections.
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