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Hypophosphatemic Rickets Workup

  • Author: James CM Chan, MD; Chief Editor: Stephen Kemp, MD, PhD  more...
 
Updated: Dec 02, 2015
 

Approach Considerations

Begin clinical laboratory evaluation of rickets with assessment of serum calcium, phosphate, and alkaline phosphatase levels. In hypophosphatemic rickets, calcium levels may be within or slightly below the reference range; alkaline phosphatase levels are significantly above the reference range.

Carefully evaluate serum phosphate levels in the first year of life, because the concentration reference range for infants (5.0-7.5 mg/dL) is high compared with that for adults (2.7-4.5 mg/dL). Hypophosphatemia can easily be missed in a baby.

Serum parathyroid hormone levels are within the reference range or slightly elevated, while calcitriol levels are low or within the lower reference range. Most importantly, urinary loss of phosphate is above the reference range.

Imaging studies

In all cases of rickets, the study of choice is radiography of the wrists, knees, ankles, and long bones. However, no pathognomonic sign on radiographs distinguishes hypophosphatemic rickets from any other etiology.

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Renal Tubular Phosphate Reabsorption

The renal tubular reabsorption of phosphate (TRP) is calculated with the following formula:

1 - [Phosphate Clearance (CPi) / Creatinine Clearance (Ccr)] X 100

The following formula calculates CPi:

[Urine Phosphate (mg/dL) X Volume (mL/min)] / Plasma Phosphate (mg/dL)

By substituting creatinine values for phosphate in the same formula, Ccr can also be calculated. A single early morning urine sample can be used, because CPi divided by Ccr causes units of urine volume to cancel each other.

The TRP in X-linked hypophosphatemia is 60%; normal TRP exceeds 90% at the same reduced plasma phosphate concentration.

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Contributor Information and Disclosures
Author

James CM Chan, MD Professor of Pediatrics, Tufts University School of Medicine; Director of Research, The Barbara Bush Children's Hospital, Maine Medical Center

James CM Chan, MD is a member of the following medical societies: American Pediatric Society, Alpha Omega Alpha, American Academy of Pediatrics, American Physiological Society, American Society of Nephrology, American Society of Pediatric Nephrology, International Society of Nephrology

Disclosure: Nothing to disclose.

Coauthor(s)

Karl S Roth, MD Retired Professor and Chair, Department of Pediatrics, Creighton University School of Medicine

Karl S Roth, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Pediatrics, American College of Nutrition, American Pediatric Society, American Society for Nutrition, American Society of Nephrology, Association of American Medical Colleges, Medical Society of Virginia, New York Academy of Sciences, Sigma Xi, Society for Pediatric Research, Southern Society for Pediatric Research

Disclosure: Nothing to disclose.

Specialty Editor Board

Mary L Windle, PharmD Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Nothing to disclose.

George P Chrousos, MD, FAAP, MACP, MACE, FRCP(London) Professor and Chair, First Department of Pediatrics, Athens University Medical School, Aghia Sophia Children's Hospital, Greece; UNESCO Chair on Adolescent Health Care, University of Athens, Greece

George P Chrousos, MD, FAAP, MACP, MACE, FRCP(London) is a member of the following medical societies: American Academy of Pediatrics, American College of Physicians, American Pediatric Society, American Society for Clinical Investigation, Association of American Physicians, Endocrine Society, Pediatric Endocrine Society, Society for Pediatric Research, American College of Endocrinology

Disclosure: Nothing to disclose.

Chief Editor

Stephen Kemp, MD, PhD Former Professor, Department of Pediatrics, Section of Pediatric Endocrinology, University of Arkansas for Medical Sciences College of Medicine, Arkansas Children's Hospital

Stephen Kemp, MD, PhD is a member of the following medical societies: American Academy of Pediatrics, American Association of Clinical Endocrinologists, American Pediatric Society, Endocrine Society, Phi Beta Kappa, Southern Medical Association, Southern Society for Pediatric Research

Disclosure: Nothing to disclose.

Additional Contributors

Arlan L Rosenbloom, MD Adjunct Distinguished Service Professor Emeritus of Pediatrics, University of Florida College of Medicine; Fellow of the American Academy of Pediatrics; Fellow of the American College of Epidemiology

Arlan L Rosenbloom, MD is a member of the following medical societies: American Academy of Pediatrics, American College of Epidemiology, American Pediatric Society, Endocrine Society, Pediatric Endocrine Society, Society for Pediatric Research, Florida Chapter of The American Academy of Pediatrics, Florida Pediatric Society, International Society for Pediatric and Adolescent Diabetes

Disclosure: Nothing to disclose.

References
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