WBC in urine

WBC are reported semiquantitatively as the number seen per high power field (HPF) using the high dry objective (40x):

none seen; <5, 5-20, 20-100, or >100/HPF

Interpretation: Less than 5 WBC/HPF is commonly accepted as normal. Greater numbers (pyuria) generally indicate the presence of an inflammatory process somewhere along the course of the urinary tract (or urogenital tract in voided specimens). Pyuria often is caused by urinary tract infection, and many times bacteria can be seen on sediment preps. Depending on clinical signs, pyuria may be an indication for culture of urine even if no bacteria are seen. Non-septic causes of inflammation, such as uroliths and tumors, also must be considered.

Identification: In regular unstained urine sediments, WBC are small cells, usually 1.5-2x larger than a RBC (see image below), are regularly round, colorless and have a slightly grainy appearance. Like erythrocytes, WBC may lyse in very dilute or highly alkaline urine. The type of WBC (neutrophil, lymphocyte, monocyte etc) cannot be determined from a regular urine wet preparation, but they are usually neutrophils. A cytologic examination would be required to determine which WBC types are present in urine, however this is rarely indicated for this purpose.

WBC must be distinguished from RBCs and small epithelial cells.

  • WBC vs RBC: The grainy nature of WBC helps to distinguish WBCs from RBCs which are smoother. WBC are also colorless whereas RBC are slightly red-tinged from hemoglobin. Identification of the segmented nuclear shape of a neutrophil is helpful, but this feature is often inapparent, particularly if the specimen is not fresh (the nucleus swells and rounds up with storage).
wbc rbc in urine
WBC (arrowheads) are larger, colorless and more granular than the redder smaller RBC (arrows), which have no internal texture (although appear slightly biconcave).
  • WBC vs small eptihelial cells: This distinction is far more difficult and may rely on cytologic examination of the urine sediment. WBCare generally smaller than epithelial cells and are more round (note that WBC can swell with storage and appear larger than normal). Epithelial cells have more angular borders or polygonal shapes and round to oval nuclei.

    wbc epi
    WBC (arrowhead) are smaller and more uniformly round compared to larger epithelial cells (likely of transitional origin) that have more angular or flatter borders (arrow).

For a compilation of images of the different cells seen in urine, see the urine sediment atlas.


Cornell University