In some cases, Pap smears may produce a false positive (incorrectly indicating a problem) or false negative (failing to indicate a problem). False negative tests may occur up to 20 percent of the time, but false positives are extremely rare. False negatives can be caused by many factors, including:

• Inadequate collection of cells
• Small number of abnormal cells
• Inaccessible location of the lesion
• Small size of the lesion
• Abnormal cells mimicking benign cells
• Blood or inflammatory cells obscuring the abnormal cells

Although abnormal cells sometimes go undetected, they are often caught in future screenings, as cervical cancer takes years to develop.