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Diagnosis dan Screening
Dr.Edward Kosasih,MARS,PA,DK
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Diagnosis dan screening
• Diagnostic and screening tests are used everyday by clinicians.
• Tests are used to
(i) make diagnoses (or increase the probability ofa diagnosis);
(ii) judge the severity of an illness; and
(iii) predict its clinical course or likely response to
treatment.Tests should be chosen based on the useful
information that they provide to clinician.
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Screening tests :
• applied to asymptomatic healthy subjects from
the general population.• An example might be population screening for
cholesterol.
Diagnostic tests :
• are used in patients with specific symptoms toexplain or investigate their most likely cause.
• are particularly useful when clinical uncertaintyexists – to 'rule in' or exclude the likelihood of a
particular diagnosis. However, sound clinical judgement about suspicious signs andsymptoms remains the best indicator for the useof diagnostic tests.
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• The 'best' test to use is a pre-determined
'gold standard'.
• The gold standard reflects the 'true'
diagnosis in that patient. In many
situations, establishing the true diagnosis
may be invasive, expensive and potentially
unrealistic – thus we often use diagnostic
tests rather than gold standard
measurements. Examples of somescreening and diagnostic tests and
possible 'gold standards' are listed below :
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Examples of screening and diagnostic tests and possible gold standards
Disease Tests Gold standard
Urinary tract infection urine microscopy urine culture
Congenital heart
disease
exercise ECG coronary angiography
Myocardial infarction ECG or cardiac
enzymes
cardiac biopsy
(only available at autopsy)
Breast cancer mammography biopsy result
Bowel cancer Hemoccult test of stool colonoscopy +/- biopsyHypertension blood pressure
(Korotkoff sounds)
intra-arterial measurement of
pressures
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Skrining
Tujuan : untuk mencegah penyakit atau akibat penyakitdengan mengidentifikasi individu2 pada suatu titikdalam riwayat alamiah ketika proses penyakit dapatdiubah melalui intervensi.
Uji skrining digunakan untuk mengidentifikasi suatupenanda awal perkembangan penyakit sehinggaintervensi dapat diterapkan untuk menghambat prosespenyakit.
Tiga tingkat pencegahan :
• Pencegahan primer
• Pencegahan sekunder
• Pencegahan tertier
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Pencegahan primer
• Ditujukan pd orang2 yg tidak memilikigejala untuk mengidentifikasi faktor risikodini penyakit guna menahan proses
patologi sebelum timbul gejala• Contoh : mengidentifikasi orang2 dalam
tahap awal gangguan toleransi glukosa,dan mengendalikan berat badan sertapola makan mereka untuk mencegahtimbulnya diabetes.
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Pencegahan sekunder
• Ditujukan pd orang2 dalam proses awal
penyakit untuk memperbaiki prognosis.
• Contoh : mengidentifikasi orang2
penginap DM yg tidak terdeteksi atau tidak
teramati untuk meningkatkan toleransi
glukosa guna mencegah penyakit
mikrovaskular yg dapat mengakibatkankerusakan ginjal, dan penyakit lainnya.
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Pencegahan tertier
• Ditujukan pada orang2 yang mengalami
komplikasi untuk mencegah dampak
lanjutan komplikasi tsb.
• Contoh : melakukan skrining pada orang2
untuk mendeteksi riwayat retinopati
diabetik agar mendapat pengobatan laser
untuk mengendalikan perdarahan retinadan mencegah kebutaan.
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Sensitivitas dan spesifitas
• Untuk mengetahui apakah suatu uji skrining
atau diagnosis dapat membedakan antara
individu yang sakit dan yang tidak sakit.
• Validitas uji ditentukan dengan membandingkanhasil uji dengan hasil yang didapat dari uji yang
lebih akurat (dikenal sebagai gold standard )
• Nilai tertentu pd hasil uji yg bersesuaian dgn
hasil gold standard menghasilkan ukuran
sensitivitas dan spesifitas.
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Sensitivitas dan spesifitas
• Sensitivitas : kemampuan suatu uji utk
memberikan hasil positif pd mereka yang
mengidap penyakit.
• Sensitivitas dinyatakan dalam persentase :
o.s. yg terdeteksi oleh uji
Jumlah seluruh o.s. yg ikut uji
X 100
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• A test which is very sensitive will rarely
miss people with the disease. It is
important to choose a sensitive test if
there are serious consequences formissing the disease. Treatable
malignancies (in situ cancers or Hodgkin's
disease) should be found early — thussensitive tests should be used in their
diagnostic work-up.
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Sensitivitas dan spesifitas
• Spesifitas : kemampuan suatu uji untuk
memberikan hasil negatif pada mereka
yang sehat (tidak sakit).
• Spesifitas juga dinyatakan dlm persentase
Org sehat yang hasil uji negatif
Jumlah seluruh orang sehat yang ikut uji
X 100
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Tabel perhitungan sensitivitas dan
spesifitas
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Contoh : hasil uji skrining DM pd 10.000 org
dengan kadar 180 mg/dl sebagai DM
Hasil uji Diagnosis yang benar
Sakit Tidak sakit Total
Positif 34 (a) 20 (b) 54 (a+b)
negatif 116 (c) 9830 (d) 9946 (c+d)
Total 150 (a+c) 9850 (b+d) (a+b+c+d)
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Perhitungan sensitivitas dan
spesifitasSensitifitas = a / a+c
= 34 /150 = 0,2266 atau 22,66%
Spesifitas = d / b+d
= 9830 / 9850 = 0,9979 atau
= 99,79%Dalam menentukan sensitifitas dan spesifitas, maka
penentuan cutoff point sangat mempengaruhi hasilperhitungan. Misalnya bila cutoff point utk diagnosis DMditurunkan menjadi 130 mg/dl, maka jumlah orang yg
tidak sakit akan berkurang, sehingga nilai spesifitasmenjadi lebih kecil dan nilai sensitivitas menjadi lebihbesar.
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• Penentuan batas nilai normal suatu kadar
kimiawi dalam darah didasarkan pada
evidence epidemiologi, misalnya nilai
cholesterol total <200 mg/dL mengurangiresiko arteriosclerosis.
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Positif palsu dan negatif palsu
• Suatu uji yang sangat sensitif memilikisedikit hasil negatif palsu (c).
• Sensitivitas = a/(a+c), maka nilai c yg kecil
akan menghasilkan sensitivitas mendekati100 %
• Suatu uji dengan spesifitas yang tinggi
memiliki sedikit positif palsu (b).• Spesifitas = d/(b+d), maka nilai b yg kecil
menghasilkan spesifitas mendekati 100%
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Contoh
• Ditentukan batas normal KGD puasa 8 mmol/L
atau PP 11 mmol/L, pada cut-points ini, maka
sensitivitas = 57% and specifitas = 99%.
• Bila cut-point = 5 mmol/L, maka sensitivitas =98%, tetapi spesifitas < 25% — banyak orang
terdiagnose diabetes secara salah (positif
palsu). Sebaliknya bila cut-point > 13 mmol/L,
maka spesifitas hampir 100%, tetapi banyakpenderita diabetes tak terdiagnosis (angka
negatif palsu sangat tinggi).
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Nilai prediktif positif
• Ad proporsi subjek dengan hasil uji positifyg benar2 sakit menurut diagnosis.
• = a / (a+b)
• Nilai prediktif positif meningkat seiringdengan meningkatnya sensitivitas danspesifitas.
•Bila prevalensi penyakit yang diskriningmeningkat, maka nilai prediktif positif jugameningkat dan sebaliknya.
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Likelihood ratio
• The likelihood ratio for a positive result
(LR+) tells you how much the odds of the
disease increase when a test is positive.
• The likelihood ratio for a negative result
(LR-) tells you how much the odds of the
disease decrease when a test is negative.
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• The Likelihood Ratio (LR) is the likelihood that a giventest result would be expected in a patient with the targetdisorder compared to the likelihood that that same resultwould be expected in a patient without the targetdisorder.
• For example, you have a patient with anaemia and aserum ferritin of 60mmol/l and you find in an article that90 per cent of patients with iron deficiency anaemia have
serum ferritins in the same range as your patient (=sensitivity) and that 15 per cent of patients with othercauses for anaemia have serum ferritins in the samerange as your patient (1 - specificity). This means thatyour patient's result would be six times as likely (90/15)
to be seen in someone with, as opposed to someonewithout, iron deficiency anaemia, and this is called theLR for a positive test result.
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Application
• The LR is used to assess how good a diagnostic
test is and to help in selecting an appropriatediagnostic test(s)
• They have advantages over sensitivity andspecificity because they are less likely to changewith the prevalence of the disorder, and can beused to calculate post-test probability for a targetdisorder.
• For example, if you thought your patient'schance of iron deficiency anaemia prior to doing
the ferritin was 50-50, this pre-test probability of50 per cent translates as pre-test odds of 1:1,and the post test odds can be calculated asfollows:
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• Post-test odds = pre-test odds * LR = 1*6 = 6
Post-test probability = post test odds / (post test
odds + 1)
= 6 / (6 + 1) = 86 per cent
• After the serum ferritin test is done and your
patient is found to have a result of 60 mmol/l, the
post-test probability of your patient having irondeficiency anaemia is therefore increased to 86
per cent, and this suggests that the serum
ferritin is a worthwhile diagnostic test.