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Introduction ⬇

Hydroxyproline is an amino acid found only in collagen. It increases during periods of rapid growth, in bone diseases, and in some endocrine disorders. Urine hydroxyproline is almost entirely peptide bound, and only 10% is in the free form.

Total hydroxyproline is considered to be a marker for bone resorption because 50% of human collagen resides in bone. This test indicates the presence of reabsorption of bone collagen in various disorders and evaluates the degree of destruction from primary or secondary bone tumors. Free hydroxyproline is used as an aid to diagnose hydroxyprolinemia, a rare genetic disorder characterized by intellectual disability and thrombocytopenia.

Normal Findings ⬆ ⬇

Urine

Total hydroxyproline (24-hour): 15–45 mg/24 hr or 115–345 μmol/d

Adult females: 0.4–2.9 mg/2-hr specimen or 3–22 μmol/2 hr

Adult males: 0.4–5.0 mg/2-hr specimen or 3–38 μmol/2 hr

Children <5 years old: 100–400 μg/mg creatinine or 86–345 mmol/d

Children 5–12 years: 100–150 μg/mg creatinine or 86–129 mmol/d

Blood (Plasma)—Free Hydroxyproline

Newborn: 0.52 ± 0.52 mg/dL or 40 ± 40 μmol/L

Child (male): <0.66 mg/dL or <50 μmol/L

Child (female): <0.58 mg/dL or <44 μmol/L

Adult (male): <0.55 mg/dL or <42 μmol/L

Adult (female): <0.45 mg/dL or <34 μmol/L

Procedure ⬆ ⬇

  1. Obtain a 2-hour specimen after the patient has fasted overnight (preferred method). Label with the patient’s name, date and time of collection, and test(s) ordered.

  2. Notify the laboratory of the patient’s age and sex.

  3. If ordered, collect a 24-hour urine specimen. No preservative is required; however, to maintain proper pH, 25 mL of HCl, 6 mol/L is added. Specimen must be refrigerated or placed on ice.

  4. Follow 24-hour urine collection procedures. The laboratory will record the total 24-hour volume.

  5. Note that the preferred method of testing in the first few months of life is blood sampling (free hydroxyproline only for genetic screening).

Clinical Implications ⬆ ⬇

  1. Free hydroxyproline is increased in:

    1. Hydroxyprolinemia, a hereditary autosomal recessive condition (very rare)

    2. Familial iminoglycinuria, also inherited and rare

  2. Total hydroxyproline is increased in:

    1. Hyperparathyroidism, hyperthyroidism

    2. Paget disease—measures the severity and the response to treatment

    3. Marfan syndrome, acromegaly

    4. Osteoporosis

    5. Myeloma

    6. Severe burns

  3. Total hydroxyproline is decreased in:

    1. Hypopituitarism

    2. Hypothyroidism

    3. Hypoparathyroidism

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain purpose of test, procedure for timed urine collection, and interfering factors. Fasting and special fluid requirements before testing are often required for a 24-hour timed procedure. Check with laboratory.

  2. Avoid gelatin- or collagen-containing foods for several days before the test.

  3. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Have the patient resume a normal diet and activity.

  2. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel patient regarding abnormal findings.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Gelatin or collagen may affect test results (false-positive test); therefore, patient should avoid ingestion of meat. For best results, the patient should be on a nonprotein diet.

  2. Bed rest increases values.

  3. Pregnancy increases values.

  4. Hydroxyproline excretion is highest at night due to diurnal variation.