A mathematical equation to differentiate overload proteinuria from tubulo-interstitial involvement in glomerular diseases.

Determination of marker proteins like albumin and alpha 1-microglobulin allows to differentiate various types of proteinuria in kidney diseases. In the present communication we calculate the degree of tubulointerstitial involvement by quantitation of the tubular marker alpha 1-microglobulin in urine in relation to albuminuria. A mathematical relation between minimal tubular proteinuria with the degree of albumin excretion was observed. Cases forming this line did not exhibit interstitial fibrosis when analyzed histologically. In contrast most cases exhibiting higher excretion rates of the tubular marker showed various degrees of tubulointerstitial involvement. In order to differentiate interstitial contribution from overload tubular proteinuria in patients with an albumin excretion rate above 3000 mg/g creatinine alpha 1-microglobulin (measured) is suggested to be corrected by the "glomerular" component of alpha 1-microglobulin using the following equation: "tubulo-interstitial alpha 1-microglobulin" = alpha 1-microglobulin (measured) -4.7 exp (2.2 x 10(-4)) [albumin]. Alternatively the correction can be performed graphically. This procedure may be of considerable help in preventing misinterpretations of urinary protein patterns in patients with nephrotic proteinuria.