First we will look at tubulointerstitial nephritis, and then we will cover pyelonephritis. Glomerulonephritis is covered in a separate lesson.

Tubulointerstitial nephritis means both tubules and interstitium are inflamed. It may be further classified according to the age of the lesion, its distribution, and the character of the cellular exudate. Describing renal lesions in this manner results in a clear concise morphologic diagnosis and aids in determining the cause of the disease.

We can determine a couple of things from examining this kidney grossly.

First, the age of the lesion is apparent. The kidney is pitted and shrunken.

This gross appearance is a reflection of severe fibrosis, which is always a CHRONIC change. We can also appreciate that the change is DIFFUSE.

Below is an example of the microscopic findings in tubulointerstitial nephritis. Microscopically, interstitial infiltrates of inflammatory cells are present including neutrophils and plasma cells and are accompanied by evidence of tubular damage or inflammation.

The morphologic diagnosis would be subacute, suppurative tubulointerstitial nephritis.

Note the inflammatory cells in the interstitium (arrows) and inflammatory cells in the tubular lumens (arrow heads).

Chronic tubulointerstitial disease progresses to interstitial fibrosis and loss of tubules. At this "endstage" it is difficult to determine what type of nephritis initiated the changes.

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