This document provides an introduction to chemical pathology (clinical biochemistry). It discusses what chemical pathology is, the rational use of laboratory tests, and purposes of laboratory tests such as screening, diagnosis, monitoring treatment, and prognosis. It also covers sources of variation in test results, including analytical sources like precision and accuracy as well as biological sources like diurnal variation. Reference ranges are described as providing the expected range of results for healthy individuals. Key metrics for diagnostic tests like sensitivity and specificity are also introduced.

1. CHEMICALP
A
THOLOGY
INTRODUCTION
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3. • WHA
TISCHEMICALPA
THOLOGY?
• Chemical Pathology is the branch of pathology
dealing with the biochemical basis of disease and
the use of biochemical tests for diagnosis and
management.
• It is also known as Clinical Biochemistry or Clinical
Chemistry
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4. RA
TIONALUSEOFLABTESTS
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6. THEUSEOFCLINICALBIOCHEMISTRYTESTS
• Laboratory tests are most often requested for
defined diagnostic purposes.
• The justification for discretionary testing is well
summarised by answering the following questions:
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7. HOWOFTENSHOULDAPA
TIENTBEINVESTIGA
TED?
This depends on the following:
• How quickly numerically significant
changes are likely to occur:
• for example, concentrations of the
main plasma protein fractions are
unlikely to change significantly in less
than a week
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8. HOWOFTENSHOULDAPA
TIENTBEINVESTIGA
TED?
• Whether a change, even if numerically
significant, will alter treatment:
• for example, plasma transaminase activities
may alter within 24 h in the course of acute
hepatitis, but, once the diagnosis has been
made, this is unlikely to affect treatment.
• By contrast, plasma potassium concentrations
may alter rapidly in patients given large doses
of diuretics and these alterations may indicate
the need to change treatment
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9. PURPOSESOFLABORA
TORYTESTS
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11. PURPOSESOFLABORA
TORYTESTS
SCREENING
Lab tests are used for mass screening (e.g.,
phenylketonuria and sickle cell in newborns),
screening asymptomatic patients (e.g.,
mammography), and
screening symptomatic patients (e.g., stress
electrocardiogram in a patient with chest pain).
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12. PURPOSESOFLABORA
TORYTESTScont’d
DIAGNOSIS
• They are used to confirm a diagnosis (e.g.,
coronary angiogram to confirm coronary artery
disease in a patient with a positive stress
electrocardiogram).
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13. PURPOSESOFLABORA
TORYTESTScont’d
MONITORINGTREA
TMENT
• They are used to monitor a patient's disease status
(e.g., serum glucose in a person with uncontrolled
diabetes).
PROGNOSIS
• providing information on disease susceptibility.
e.g. prognosis can be predicted by noting the
degree of test abnormality
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14. Test selection for the purpose of discretionary testing
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15. LABORA
TORYSCREENINGCRITERIA
1. There must be a high prevalence of the disease to
justify the expense.
2. Significant morbidity and mortality must be
associated with the disease if it is left untreated.
3. The disease must be detectable before symptoms
surface in the patient.
4. An effective therapy must be available that is safe
and inexpensive.
5. The test must be cost effective and easily
performed in the laboratory.
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16. SEQUENCEOFTESTS
• Depends on several factors
1. Situation critical: test with the highest yield is
done, even though there may be some risks
2. If there is time: lower yield, less risky procedure
done first
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17. ORDEROFTESTING
1. From cheap to costly
2. From less to more risky
3. From simple to more complex
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18. ORDEROFTESTINGcont’d
Not always practical.
• One or more objectives may be sacrificed for
speed, convenience, accuracy, a waiting list for
procedures, time needed to await the results, and
the condition of the patient.
• Sometimes it may be best to get the costly test
done first; it may solve the problem quickly and
save money in the long run.
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19. SOURCESOFV
ARIA
TIONINTESTRESULTS
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20. What are the indicators of test reliability?
• Accuracy, Precision, Specificity and Sensitivity
• Accuracy and precision reflect how well the test
method performs day to day in a laboratory.
• Sensitivity and specificity deal with how well the
test is able to distinguish disease from absence of
disease.
• These are effectively analytical sources of
variation.
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21. PRECISIONANDACCURACYIN
BIOCHEMICALTESTS
• Precision: The amount of variation in results after
measuring the same sample repeatedly.
• A test method is said to be precise when repeated
analyses on the same sample give similar results.
• Accuracy: How close the result is to the "true" value as
determined by a reference method.
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24. • Although a test that is 100% accurate and 100%
precise is ideal, in practice, test methodology,
instrumentation, and laboratory operations all
contribute to small but measurable variations in
results.
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25. BIOLOGICALCAUSESOFV
ARIA
TION
• As well as analytical variation, test results also
show biological variation in both health and
disease.
• Key questions are:
• How do results vary in health?
• How do results vary in disease?
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26. How do results vary in health?
• The concentrations of all analytes in blood vary
with time due to diverse physiological factors
within the individual.
• There are also differences between individuals.
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27. Within-individual variation
• The following may be important causes of within
individual variation:
• DIET: Variations in diet can affect the results of
many tests, including serum triglyceride
• TIME OF DAY: Several plasma constituents show
diurnal variation (variation with the time of day),
or a sleep/wake cycle. Example is cortisol
• POSTURE: Proteins and all protein-bound
constituents of plasma show significant
differences in concentration between blood
collected from upright individuals and blood from
recumbent individuals.
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28. Within-individual variation cont’d
• MUSCULAR EXERCISE: Recent exercise, especially
if vigorous or unaccustomed, may increase serum
creatine kinase (CK) activity in blood [lactate],
and lower blood [pyruvate].
• MENSTRUALCYCLE: Several substances show
variation with the phase of the cycle. Examples
include serum [iron], and the serum
concentrations of the pituitary gonadotrophins
• DRUGS: These can have marked effects on
chemical results.
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29. RESUL
TSV
ARIA
TION
SERUM CREATININE [umol/L] IN FOUR APPARENTLY
HEALTHY INDIVIDUALS IN SIX SAMPLES TAKEN AT DAILY
INTERVALS
Sample Subject 1 Subject 2 Subject 3 Subject 4
1 86 124 97 144
2 81 128 93 139
3 82 120 95 141
4 84 126 91 138
5 80 130 97 146
6
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30. Case Study
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31. Between-individual variation
• Differences between individuals can affect the
concentrations of analytes in the blood. The
following are the main examples:
• AGE: Examples include serum [phosphate] and
alkaline phosphatase (ALP) activity
• SEX: Examples include serum creatinine, iron,
urate and urea concentrations
• RACE: Racial differences have been described for
serum [cholesterol] and [protein].
It may be difficult to distinguish racial from
environmental factors, such as diet
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32. How do results vary in disease?
• Biochemical test results do not exist in isolation.
• For example, in a patient with severe abdominal
pain, tenderness and rigidity, there may be several
differential diagnoses to consider – including, for
example,
• acute pancreatitis
• perforated peptic ulcer and
• acute cholecystitis.
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33. ANAL
YTICALSENSITIVITYAND
SPECIFICITY
• The analytical sensitivity of an assay is a
measure of how little of the analyte the
method can detect.
• The assay’s ability to detect very low
concentrations of a given substance in a
biological specimen.
• Analytical sensitivity is often referred to as
the limit of detection (LoD).
• LoD is the actual concentration of an analyte
in a specimen that can be consistently
detected ≥ 95% of the time.
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34. ANAL
YTICALSPECIFICITY
• Analytical specificity of an assay relates to how
good the assay is at discriminating between the
requested analyte and potentially interfering
substances.
• The assay’s ability to detect the intended target.
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35. REFERENCERANGES
• Reference intervals/ ranges are fundamental tools
used by medical practitioners to interpret patient
laboratory test results and help differentiate
between healthy and unhealthy individuals.
• Also referred to as “normal” or “expected” values,
reference intervals provide the range of
laboratory test results that would be expected in
a healthy population.
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36. REFERENCERANGEScont’d
• Results are therefore interpreted by comparing
with a set of results from a particular defined (or
reference) population.
• To interpret results on patients adequately, we
need to know:
• the reference range for healthy individuals of the
appropriate age range and of the same sex;
• the values to be expected for patients with the
disease, or diseases, under consideration
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37. REFERENCERANGEScont’d
• Biochemical test results are usually compared
to a reference interval chosen arbitrarily to
include 95% of the values found in healthy
volunteers.
• This means that, by definition, 5% of any
population will have a result outside the
reference interval
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41. MEANINGOFNORMALAND
REFERENCERANGES
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42. • Diagnostic/Test Sensitivity
• Sensitivity is the ability of a test to correctly
identify individuals who have a given disease or
condition.
• For example, a certain test may have proven to be
90% sensitive
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43. • Diagnostic/Test Specificity
• Specificity is the ability of a test to correctly
exclude individuals who do not have a given
disease or condition.
• For example, a certain test may have proven to be
90% specific
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44. • sensitivity (the ability of a test to detect a disease
when it is present), and specificity ( the ability of a
test to reflect the absence of the disease in those
disease-free) can be calculated as follows:
• Specificity= 100 x TN / (TN + FP) %
• Sensitivity = 100 x TP / (TP + FN) %
• Where,
• TN=True negative
• TP= True positive
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45. PERFECTTEST
• Accurate
• Precise
• Discriminating
• Pain free
• Risk free
• Inexpensive
• Useful
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46. ACCURACYANDPRECISIONcont’d
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47. • In Fig above, both graphs show the distribution of
results for repeated analysis of the same sample by
different methods.
• Methods A and B on the left are equally accurate (mean
value is the same) but the lesser scatter in A makes it
more precise.
• C and D on the right are equally precise, but in D the
mean value differs from the true, so C is more accurate.
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50. CASESTUDY
• A sample from a 65-year-old male arrived in the
laboratory at 9 am from a well-man clinic. The
potassium is 8.5 mmol/L which is dangerously high.
The sample is repeated and the same result is
obtained. All other laboratory checks have been
carried out and the result is analytically valid. The
result is telephoned to the GP who reveals that the
sample was taken at 4 pm the previous day by the
nurse.
(a) What is the most likely cause of the high result?
(b) What would your recommended course of action
be?
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