Most Sensitive Sign for Pneumonia
Increased respiratory rate (tachypnea) is the most sensitive clinical sign for pneumonia in this patient, with a respiratory rate >50 breaths/min showing the highest diagnostic accuracy among individual clinical findings.
Evidence Supporting Tachypnea as Most Sensitive
The guideline evidence consistently demonstrates that tachypnea outperforms other individual clinical signs in sensitivity for detecting pneumonia:
Respiratory rate >50 breaths/min has a positive likelihood ratio of 1.90 (95% CI 1.45–2.48), which was the highest among all individual clinical symptoms and signs examined against radiologically confirmed pneumonia 1.
In pediatric populations specifically, the presence of either respiratory rate ≥50/min or chest indrawing showed a predictive power of 46% for a positive test and 83% for a negative test, making it a reliable indicator 2.
Abnormal vital signs (including respirations >20/min in adults) were 97% sensitive for detecting pneumonia in emergency department patients, far exceeding the sensitivity of any other individual finding 3.
Why Other Signs Are Less Sensitive
Cough (Option A)
- While cough is the most common symptom in pneumonia patients (86%), it is equally common in those with other respiratory illnesses, making it non-specific and poorly predictive 3.
- Cough lacks discriminatory power as a standalone sign 1.
Fever (Option B)
- Fever was absent in 31% of patients with confirmed pneumonia, demonstrating poor sensitivity 3.
- The evidence explicitly states that fever is non-specific and variably present in pneumonia 1.
- Temperature ≥38°C is suggestive but not reliably sensitive as an isolated finding 1.
Dyspnea (Option D)
- While dyspnea is an important symptom, it was not identified as having superior sensitivity compared to tachypnea in the systematic reviews 1.
- Dyspnea is more subjective and less reliably measured than respiratory rate 1.
Important Clinical Context
No single clinical criterion has both high sensitivity and specificity for pneumonia 1. The evidence shows significant heterogeneity across studies, and all individual signs have limitations:
- Twenty-two percent of patients with completely normal chest examinations had confirmed pneumonia 3.
- The combination of clinical findings performs better than any single sign, but when forced to choose the most sensitive individual sign, tachypnea consistently demonstrates the highest diagnostic utility 1, 3, 2.
Practical Application
In your patient with RR 50:
- The elevated respiratory rate is the most reliable single indicator that pneumonia is present 1, 3, 2.
- This finding should prompt chest radiography and consideration of empiric antibiotics according to local guidelines 1.
- Tachypnea combined with abnormal breath sounds has a high negative predictive value (97%), making the absence of these findings useful for ruling out pneumonia 4.
Answer: C. Increased respiratory rate (Tachypnea)