Why to Use
The Rule of 7s has been validated by a retrospective cohort study of 423 children in Lyme-endemic areas. It can help guide clinicians in assessing the need to initiate antibiotic therapy for Lyme meningitis, versus observation and close follow-up care.
When to Use
Use the Rule of 7s in Lyme-endemic areas when considering antibiotic treatment for pediatric patients who:
Abbreviations: CSF, cerebrospinal fluid; RBC, red blood cell; WBC, white blood cell.
This tool should be used to assist clinicians in decision-making, not to replace clinical evaluation of a patient. Patients with scores of 1 to 3 points are not at low risk for Lyme meningitis, and antibiotic therapy for Lyme meningitis should be considered for these patients. Patients with a score of 0 are at low risk for Lyme meningitis; their symptoms may be due to aseptic meningitis or another etiology. Clinicians should use clinical judgment and consider whether the patient has access to adequate follow-up care before initiating antibiotic therapy.
Matthew Lecuyer, MD
The Rule of 7s is meant to aid in the decision to begin antibiotic therapy for suspected Lyme meningitis. It should not replace clinical judgement and clinician assessment of patients.
Avery et al (2006) first derived a clinical prediction model to calculate the probability of Lyme meningitis in children from Lyme-endemic regions, using a statistical analysis of history, physical examiniation, and laboratory findings. Their model was prospectively validated by Garro et al (2009) in a study of 50 children aged 2 to 18 years who lived in a Lyme-endemic region. Fourteen of the children had Lyme meningitis, 6 had possible Lyme meningitis, and 30 were ultimately diagnosed with aseptic meningitis. Categories of low (< 10%), indeterminate (10%-50%), and high (> 50%) probabilities of Lyme meningitis were derived based on the percentage of CSF mononuclear cells, duration of headache, and presence of cranial nerve neuropathy.
The positive predictive value with a cutoff of > 50% probability of Lyme meningitis was 100% (95% confidence interval [CI]: 66%-100%). The negative predictive value with a cutoff of < 10% probability of Lyme meningitis was 100% (95% CI: 82%-100%). The authors noted that when patients had < 7 days of headache, < 70% CSF mononuclear cells, and no seventh or other cranial nerve palsy, the probability of Lyme meningitis was always < 10%, indicating that those patients were at low risk for Lyme meningitis. The authors termed this the Rule of 7s.
The Garro et al study was validated in a large retrospective cohort study by Cohn et al (2012) using electronic medical record data from 3 pediatric emergency departments in Lyme-endemic areas. The sample of 423 children, aged 90 days to 19 years, included 117 children who were diagnosed with Lyme meningitis and 306 who were diagnosed with aseptic meningitis. The specificity of the Rule of 7s for low risk was 41% (95% CI: 36%-47%), and the sensitivity was 96% (95% CI: 90%-99%).
Aris Garro, MD, MPH
Copyright © MDCalc • Reprinted with permission.
Jennifer Bellis, MD, MPH; Ee Tay, MD
Michael Gottlieb, MD, RDMS; Lise Nigrovic, MD, MPH
September 2, 2018