Bone

bone
- 65% minerals (hydroxyapatite)
- 35% matrix
  - Collagen
  - Osteoblasts
  - Osteoclasts

Markers
- High biological and analytical variability
  - Diurnal variation
  - Seasonal variation
  - Food intake
  - Exercise
- Made in same lab, collected at same time
Markers of bone formation
- Total alkaline phosphatase
- Bone specific alkaline phosphatase
- Osteocalcin – increased in high bone turnover
- Procollagen propeptides
  - PICP – procollagen type I C-terminal
  - PINP – procollagen type I N-terminal

Markers of bone resorption
- Urine (corrected for creatinine)
  - Deoxypyridinoline (cross-linking collagen cleavage products)
  - N-terminal cross links (NTX) – N-telopeptide of type I collagen
  - C-terminal cross links (CTX) – C-telopeptide of type I collagen
- Serum
  - TRACP5b – tartate resistant acid phosphatase
- Serum and urine
  - beta-crosslaps

Osteoporosis

Osteoporosis refers to the decreased bone density, which increases the risk for fractures. It’s defined according to the results of a DEXA (dual-energy x-ray absorptiometry) scan or by the presence of a pathological fracture. The DEXA scan calculates the bone mineral density and gives a T-score. Osteoporosis is defined as a T-score of less than -2,5 standard deviations. This means that a person with osteoporosis has 2,5 standard deviations lower bone density than an average young adult female.

T-score between -1 and -2,5 is less severe and is called osteopaenia.

Etiology

Female gender
Old age
Smoking
Low calcium intake
Low vitamin D intake
Low levels of physical exercise
Alcohol consumption

Osteoporosis can also be secondary to:

Diagnosis and evaluation

Even though osteoporosis is defined based on the DEXA scan, it can be diagnosed in patient with risk factors who present with pathological fractures. In the absence of pathological fractures, a DEXA scan is required for diagnosis. Even though the pathological fracture is enough to make the diagnosis, a DEXA scan of these patients is usually performed anyway as part of the evaluation.

Some countries recommend screening for osteoporosis in persons at risk, like postmenopausal persons above a certain age, or patients on long-term glucocorticoid therapy. In other countries (like Norway) patients are only evaluated after a pathological fracture has occurred.

Biochemical bone markers

Multiple markers can be measured in the blood which reflect bone metabolism. Osteocalcin, bone-specific alkaline phosphate, and N-terminal propeptide type 1 collagen (P1NP) are markers of bone formation, while C-terminal telopeptide type 1 collagen (CTX-1) is a marker of bone resorption. They can be measured in the urine, but serum measurement is more convenient. These markers are mostly used to evaluate treatment response.