si unit & nilai rujukan

SI Unit dan Nilai Rujukan

SI Units &Reference INTERVALSJanuarti SururiPembimbing: dr. Yusra, SpPK, PhDSome laboratories may report test results using different units from other laboratories universal units needed so that the reports can be understood by and compared with, internationally. Si uNITSi uNIT1873 : CGS (centimeter, gram, second) system1901 : MKS (meter, kilogram, second) system1950 : MKSA (meter, kilogram, second, ampere) extended to SI units = International system of units = Systme International dUnits1960 : SI units adopted by > 50 countries at 11th Confrence Gnraldes Poids et Mesures (CGPM)1966 : International Federation of Clinical Chemistry (IFCC) recommended the use of SI units for clinical laboratory

Van Assendelft OW. The international system of units (SI) in historical perspective. American Journal of Public Health. 1987;77(11):1400-1403.

Two classes of units:(1) Base units (2) Derived units: Coherent units - derived directly from the base units without using conversion factors Noncoherent units - constructed from the base units and contain a numerical factor to make the numbers more convenient to use.

Lehmann HP, Henry JB. SI Units. McPherson RA, Pincus RR. Henrys Clinical Diagnosis and Management by Laboratory Methods 22nd Edition. Philadelphia: WB Saunders Co, 2011 p 1491.

Si uNITSi uNIT Lehmann HP, Henry JB. SI Units. McPherson RA, Pincus RR. Henrys Clinical Diagnosis and Management by Laboratory Methods 22nd Edition. Philadelphia: WB Saunders Co, 2011 p 1491.

Si uNIT

Lehmann HP, Henry JB. SI Units. McPherson RA, Pincus RR. Henrys Clinical Diagnosis and Management by Laboratory Methods 22nd Edition. Philadelphia: WB Saunders Co, 2011 p 1491..

Si uNIT

Lehmann HP, Henry JB. SI Units. McPherson RA, Pincus RR. Henrys Clinical Diagnosis and Management by Laboratory Methods 22nd Edition. Philadelphia: WB Saunders Co, 2011 p 1491.

Si uNITAdvantages of SI units in clinical use: Universality of measurement units worldwide (reports, studies, journals, textbooks) Adequate and unambiguous expression of measurement Making it easier to compare measurement results between labs

However, some conventional units are still used until now, eg: Enzyme units conventional U/L coherent SI unit katal (for catalytic activity, incl. enzymes) Katal : the number of moles of substrate converted per second under defined conditions

H Lehmann HP, Henry JB. SI Units. McPherson RA, Pincus RR. Henrys Clinical Diagnosis and Management by Laboratory Methods 22nd Edition. Philadelphia: WB Saunders Co, 2011 p 1491..

REFERENCE INTERVALSLab tests to detect, diagnose, or monitor disease, or predisposition to disease test results interpretation medical decision

Test results Normal values = disease-free? Abnormal test results = diseased?

Reference Values/ Intervals

H Lehmann HP, Henry JB. Post Analysis: Medical Decision Making. McPherson RA, Pincus RR. Henrys Clinical Diagnosis and Management by Laboratory Methods 21nd Edition. Philadelphia: WB Saunders Co, 2007 p 68-75

REFERENCE INTERVALSA reference interval is defined as the range of values that represents the central 95% tendency of measurements from a population of non-diseased or normal individuals.

An attempt to distinguish normal from abnormal patient populations help clinicians interpret results & take decisions. Eg: reference interval has 2 cut-offs: lower end, upper end low values (abnormal), normal values (normal), high values (abnormal)

H Lehmann HP, Henry JB. Post Analysis: Medical Decision Making. McPherson RA, Pincus RR. Henrys Clinical Diagnosis and Management by Laboratory Methods 21 nd Edition. Philadelphia: WB Saunders Co, 2007 p 68-75

REFERENCE INTERVALSRandom variability

What accounts for the spread of a reference interval:1. Analytical variability imprecision2. Biological variability - intra-individuals -> biological changes over time, (eg: kortisol, vitamin D), food intake, exercise - inter-individuals -> age, sex, disease


REFERENCE INTERVALSIdeally, a distribution of test results from normal individuals would be completely distinct from abnormal individuals Test result reflects with certainty whether disease is present or absent. Unfortunately, even near perfect tests are not ideal


REFERENCE INTERVALSTests yield a continuum of results and some overlap of values between individuals with and without disease. In the overlap area, the tests cant discriminate disease from no disease migh cause false results


REFERENCE INTERVALSFalse positive -> inapproppriate admission/ therapyFalse negative -> undertreatment, transmission of infection, death


Sensitivity & specificitySensitivity: the ability of a test to detect disease Specificity: the ability of a test to detect absence of disease Two key parameters describing accuracy of a test Altering cutoff changes a tests sensitivity & specificity lowered cutoff increased sens, decreased spec raised cutoff increased spec, decreased sens

H Lehmann HP, Henry JB. Post Analysis: Medical Decision Making. McPherson RA, Pincus RR. Henrys Clinical Diagnosis and Management by Laboratory Methods 21 nd Edition. Philadelphia: WB Saunders Co, 2007 p 68-75Sensitivity & specificityROC plot: a graphic representation of the varying sensitivities & specificities that are possible by varying a tests cutoff.


Sensitivity & specificityWhich one is the priority: sens or spec?Screening test with high sensitivity, so we will not miss any diseaseDiagnostic test with high specificity, so we can be sure of the diagnosis


si unit & nilai rujukan

Documents