Negative Results – Why?

Does not necessarily mean the pathogen is not present; is just not present in that specimen?

• Specimen collected from incorrect site 
• Cells not present – require cells as viruses are in cells 
• Specimen collected from patient too late – viral shedding has ceased
• Specimen size is too small (not enough for detection)/too large (pathogen diluted out)
• Specimen integrity compromised
• Pathogen viability compromised due to inappropriate processing, handling or storage 
• Agent is an emerging pathogen not detectable by current laboratory methods (primer sites)