1. A new diagnostic test (“Cyanotest”) is being evaluated to rapidly determine the presence of cyanide toxicity (Gold Standard: Whole blood cyanide concentration > 40 micromol/L)1 in patients who are house fire victims. All patients in the study have whole blood cyanide concentrations obtained upon arrival to the hospital at the same time the Cyanotest is performed. When tested in a multicenter international study of patients with suspected cyanide toxicity, the Cyanotest provides the following results:
There are a total of 100 people in the study (n = 100).
a. 10 people test positive on the Cyanotest; 6 people who test positive on the Cyanotest are diseased (i.e., have whole blood cyanide concentration > 40 micromol/L)
b. 90 people test negative on the Cyanotest; 46 of those who test negative on the Cyanotest are not diseased (i.e., have whole blood cyanide concentration < 40 micromol/L)
Question: What is the sensitivity and specificity of the Cyanotest?
To determine the sensitivity and specificity (i.e., test characteristics) of the Cyanotest, the following 2 x 2 table is generated (see reference 2): Key: Cyanotest (+): positive Cyanotest; Cyanotest (-): negative Cyanotest; Disease (+): Cyanide poisoned (whole blood cyanide concentration > 40 micromol/L); Disease (-): Not cyanide poisoned (whole blood cyanide concentration < 40 micromol/L); TP: True Positives; TN: True Negatives; FP: False Positives; FN: False Negatives
Sensitivity = True Positives / (True Positives + False Negatives)
6/10 = 60%
Specificity = True Negatives / (True Negatives + False Positives)
46/90 = 51%
Interpretation: The Sensitivity (or proportion of patients who test positive on the Cyanotest when they have a whole blood cyanide concentration > 40 micromol/L) is 60%.
The Specificity (or proportion of patients who test negative on the Cyanotest when they have a whole blood cyanide concentration < 40 micromol/L) is 51%.
1. Baud FJ, Barriot P, Toffis V et al. Elevated blood cyanide concentrations in victims of smoke inhalation. N Engl J Med 1991; 325:1761-1766
DOI: 10.1056/NEJM199112193252502
2. Sahagún, BE, Williams C, Su MK. Biostatistics and Epidemiology Principles for the Toxicologist: The “Testy” Test Characteristics Part I—Sensitivity and Specificity. J Med Toxicol. 2022 Nov 23. doi: 10.1007/s13181-022-00916-0. Online ahead of print. PMID: 36422827
Contributed by: Mark Su, New York City Poison Control Center