Length of polycystic kidney may predict disease course

By Larry Hand

NEW YORK (Reuters Health) - Predicting the course of autosomal dominant polycystic kidney disease (ADPCKD) may require only an ultrasound measurement of kidney length, rather than the standard assessment of total kidney volume, researchers say.

"Using US kidney length to predict future chronic kidney disease (CKD) stage 3 in ADPKD will reduce costs while providing the necessary prognostic information regarding potential outcomes and complications related to ADPKD," Dr. Arlene B. Chapman and colleagues wrote online April 1 in Kidney International.

"What this study helps clinicians to do is identify polycystic kidney disease in patients who are at risk of progressing to renal failure and individuals who are going to be relatively asymptomatic for the rest of their lives. It really looks like imaging of the kidneys very quickly ascertained two specific groups of patients," Dr. Chapman told Reuters Health in a telephone interview.

Dr. Chapman is now at the University of Chicago. When she was still at Emory University in Atlanta, she and her colleagues compared simultaneous US and MRI imaging in 241 patients, ages 15-46, with ADPCKD and creatinine clearance of 70 ml/min.

They compared US and MRI kidney length (KL) and total kidney volume (TKV) using Bland-Altman plots and intraclass correlations.

Participants had repeat clearance measurements over a mean of 9.3 years, and clinicians used each measure to predict future CKD stage 3. Of the 242 patients, 202 (83.8%) had an iothalamate clearance measured at a minimum of six years (to calculate glomerular filtration rate), and 182 had done so at eight years.

"Bland-Altman plots demonstrated high levels of agreement between US and MRI kidney length, as did scatter plots," the researchers wrote. Mean paired difference or bias of US-MRI came to 44.2 ml for the right and 44.9 ml for the left kidney. Height-adjusted total TKV mean difference or bias came to 0.39 cm for right and 0.51 cm for the left kidney (p<0.01 for both).

Intraclass correlations (ICC) showed relatively strong agreement between US and MRI, with ICC for both genders turning out to be 0.828 for the right and 0.87 for the left kidney (p<0.01). Men had the strongest ICC of 0.849.

Researchers found the optimal cut points between severities to be 650 ml for US and 550 ml for MRI based on specificity and sensitivity and 16.5 cm and 16.0 cm for US and MRI, respectively, for kidney length. This shows US and MRI measurements to be similar in kidneys of normal to moderate size (<17 cm), the researchers wrote.

"The high level of accuracy of US when kidney length is <17 cm is most likely because of the fact that the entire kidney can be obtained in a single view," the researchers explained.

"I think everyone with ADPCKD when first taken care of should have someone assess their kidney length. That's an important message," she added. "Clinicians have had images given to them. They may not have had the kidney length given to them, but including that measurement in the US report by radiologists will be extremely helpful."

While MRI measurements of TKV are highly accurate and reproducible, the process is time-consuming, expensive, and not available at all healthcare centers, the researchers wrote. On the other hand, US is widely available, relatively low cost, and carries a lesser risk of radiation exposure.

The costs vary according to institution and insurance companies, Dr. Chapman said, but the cost of US could represent up to a ten-fold reduction from MRI costs.

SOURCE: http://bit.ly/1CfA7Na

Kidney Int 2015.

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