investigations

Last reviewed 01/2018

Biochemistry:

• reduced serum calcium and phosphate
• increased alkaline phosphatase
• decreased urinary calcium
• calcium-phosphate product, ie the calcium and phosphate concentrations in mmol/l multiplied together, is diagnostically less than 2.4; normal value is 3.0
• the level of 25 OH vitamin D is the best indicator of disease (1)
  • concentration of 25OHD below around 25nmol/L (10µg/L) is probably consistent with vitamin D deficiency in children, in which both clinical and biochemical abnormalities (e.g. rickets or symptomatic hypocalcaemia) may occur.
  • concentrations of 30-50nmol/L (12-20µg/L) are frequently associated with biochemical disturbances (particularly raised PTH, which is considered a sign of vitamin D insufficiency), but not clinical problems
  • if there is any suspicion that a child has vitamin D deficiency or rickets, the GP should measure serum concentrations of 25OHD and send the child for appropriate X-rays. If a child is found to have rickets then s/he should be referred to a paediatrician for further monitoring and treatment

Radiology:

• Looser's zones are characteristic but not always present
• commonly there is a picture of general skeletal deformity which may include:
• crush fractures of the vertebrae
• trefoil pelvis
• spontaneous fractures of the ribs, pubic rami, femoral neck or the metaphyses above and below the knee
• bowing of the long bones
• in children - increased depth and width of the epiphyseal growth plate, and a 'cupped' appearance of the adjacent metaphyses

Bone biopsy:

• decreased rate of bone turnover with excessive unmineralised osteoid - the biopsy is normal in osteoporosis

Reference:

1. Drug and Therapeutics Bulletin 2006; 44 (2):12-16.