Laboratory Tests in Clinical Diagnosis
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Questions and Answers

Why is the calculated osmolarity of plasma significantly lower than the true osmolality?

  • Due to the low concentration of sodium in plasma
  • Due to the incomplete ionization of sodium and potassium
  • Due to the high concentration of proteins in plasma (correct)
  • Due to the high concentration of urea in plasma

    • What is the effect of increased protein concentration in plasma on the volume of solvent?

  • It reduces the volume of solution
  • It has no effect on the volume of solvent
  • It decreases the volume of solvent (correct)
  • It increases the volume of solvent

    • What is the unit of measurement of the concentration of ions in plasma according to most methods?

  • Parts per million (ppm)
  • Molality (mmol/kg)
  • Normality (eq/L)
  • Molarity (mmol/L) (correct)

    • Why is the molarity of certain ions such as sodium reduced in pseudohyponatraemia?

    <p>Due to the increased concentration of proteins in plasma</p> Signup and view all the answers

    What is the purpose of the factor of 2 in the formula for calculating plasma osmolality?

    <p>To allow for the associated anions and assume complete ionization</p> Signup and view all the answers

    What is the effect of increased protein concentration in plasma on the molality of certain ions?

    <p>It has no effect on the molality of certain ions</p> Signup and view all the answers

    What is the term used to describe the apparently low sodium concentration in plasma due to high protein concentration?

    <p>Pseudohyponatraemia</p> Signup and view all the answers

    What is the purpose of osmometers in measuring plasma osmolality?

    <p>To measure changes in the properties of a solution</p> Signup and view all the answers

    What is the formula used to calculate plasma osmolality?

    <p>Plasma osmolality = 2[Na+] + 2[K+] + [urea] + [glucose] in mmol/L</p> Signup and view all the answers

    Why is the volume of solution greater than the volume of solvent in a protein solution?

    <p>Due to the high concentration of proteins in the solution</p> Signup and view all the answers

    Study Notes

    Requesting Laboratory Tests

    • Laboratory tests should be used judiciously to provide useful information, avoid unnecessary discomfort, and optimize patient care.
    • Overinvestigation can be harmful, causing delay in treatment, and wasting resources.
    • Before requesting a test, clinicians should consider whether its result would influence their clinical management of the patient.

    When is a Laboratory Investigation 'Urgent'?

    • A laboratory investigation is considered 'urgent' if an early answer will alter the patient's clinical management.
    • Laboratory staff should be consulted and the sample 'flagged' as clearly urgent if the test is required immediately.

    Interpreting Results

    • When interpreting laboratory results, clinicians should ask:
      • Is the result the correct one for the patient?
      • Does the result fit with the clinical findings?
      • Is the result abnormal when the appropriate reference range is taken into account?
      • Is the abnormality of diagnostic significance or a non-specific finding?
      • If it is one of a series of results, has there been a change and, if so, is this change clinically significant?

    Diagnostic Performance

    • Test sensitivity measures the frequency of a test being positive when a particular disease is present.
    • Test specificity measures the frequency of a test being negative when a certain disease is absent.
    • Receiver operating characteristic (ROC) curves visually express the usefulness of tests.
    • The predictive value of a negative result is the percentage of all negative results that are true negatives.

    Test Reference Ranges

    • A reference range (or interval) usually includes 95 per cent of the test results obtained from a healthy and sometimes age- and sex-defined population.
    • For most tests, the individual's results for any constituent are distributed around the mean in a 'normal' (Gaussian) distribution.
    • The 95 per cent limits are about two standard deviations from the mean.

    Pseudohyponatraemia

    • Pseudohyponatraemia occurs when the concentration of proteins in plasma is markedly increased, reducing the volume of solvent but not the volume of solution.
    • This can cause the molarity (mmol/L) of certain ions, such as sodium, to be reduced, but the molality remains unchanged.
    • A formula can be used to calculate plasma osmolarity, taking into account the concentrations of sodium, potassium, urea, and glucose.

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    Description

    This quiz covers the importance of laboratory tests in diagnosis, including when to request urgent tests and how to interpret results.

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