Lab medicine Flashcards
What is the definition of sensitivity ?
Sensitivity is defined as the proportion of the people with the target disease who test positive.
A sensitive test is used to ?
exclude or rule out the disease
How to mathematically calculate sensitivity ?
TP/ TP+ FN
Specificity is defined as ?
Specificity is the proportion of people WITHOUT the target Disease that have a NEGATIVE blood test.
A specific test is used to ?
A specific test is used for ruling in a disease, as it rarely misclassifies those WITHOUT a disease as being sick
How to mathematically calculate specificity ?
TN/ TN+ FP
definition of positive predictive value
Positive Predictive Value (PPV) is the proportion of those with a POSITIVE test that have the target disease in a population.
Mathematical estimation of positive predictive value ?
TP / TP+FP
Definition of negative predictive value ?
Negative Predictive Value (NPV) is the proportion of those with a NEGATIVE blood test that do not have the target disease in a population.
Mathematical estimation of negative predictive value ?
TN/TN+FN
What is the limitation of predictive values?
The positive and negative predictive values are specific to the population in questions. Therefore, comparison of predictive values with an incongruent population will be meaningless.
What are the types of Urine Studies
- Rapid urine test (dipstick)
- Urinalysis
- Urine culture
- 24-Hr urine collection
- Urine drug screen / urine toxicology screen
- Pregnancy test
What are the components of Urine dipstick ?
- pH – dehydration, diabetes, infection, acid/base d/o, kidney disease
- Specific gravity - dehydration
- Glucose - diabetes
- Ketone - diabetes
- Nitrite - infection
- Leukocytes - infection
- Protein – kidney disease, diabetes, HTN
- Blood – kidney disease, infection, stones, prostate dz, bladder dz/CA
- Bilirubin – liver disease, gall bladder disease
- Urobilinogen – liver disease, haemolytic anaemia
What are the steps of urinalysis ?
- Gross inspection
- test strip
- Microscopy
Gross inspection looks for
- Color
*Turbidity - Odour
Purpose of urinalysis ?
To look for microbes, urine casts in the context of kidney disease and infections, to look for crystals and creatinine.
What is the definition of urine specific gravity ?
USG is defined as the ratio between the density of urine and the density of an equal volume of pure distilled water.
Normal urine specific gravity ?
1.016 to 1.022
Normal urine volume ?
1000 to 1600 ML/Day
Oliguria
< 500 cc/day
Anuria
<100 cc /day
Polyuria
> 2500 to 3000 cc/day
Normal urine PH
4.5 to 6.5
Normal Glomerular capillaries are permeable to molecules of what size ?
<20,000 dalton.
what is the purpose of 24 Hr urine collection ?
It is used detect adrenal diseases by detecting adrenal hormones in urine/ kidney diseases, parathyroid gland diseases.
urine drug screen is a qualitative study: commonly screened drugs are ?
- Amphetamines (1-4d)
- Barbiturates (2-4d)
- Benzodiazepines (1-30d)
- Cannabinoids (1-30d)
- Cocaine (2-7d)
- Opiates (1-6d)
- Phencyclidine (4-30d)
Urine pregnancy test ?
test for Beta human chorionic gonadotropin levels produced by the placenta.
What are Cardiac Troponin T (cTnT) and HS-Cardiac Troponin T (cTnT)?
They are highly sensitivity markers of acute cardiac injury and they peak very early in the blood than most other markers.
Explain why brain natriuretic peptide (BNP) will be elevated in periods of exacerbation of underlying heart failure ?
BNP and NT-proBNP are reflexively released by the ventricular cardio-myocytes in response to volume and pressure overload as seen in CHF.
How BNP and NT-proBNP can be used to evaluate heart failure?
an increase BNP >100 pg/L is indicative of CHF.
NT-proBNP level > 450 pg/L in people <50, >900 pg/L between 50-75 years, and > 1800 pg/L in people >75 is suggestive of CHF.
What are the components of liver profile ?
*alanine aminotransferase (ALT), *aspartate aminotransferase (AST), alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), *5’nucleotidase,
*total bilirubin: conjugated (direct) bilirubin, unconjugated (indirect)bilirubin
*prothrombin time (PT)
*the international normalized ratio (INR)
*lactate dehydrogenase
*total protein
*globulins
*albumin.
* G/A ratio
Albumin/Globulin ratio higher or lower than normal are associated with ?
liver and/or kidney disease
hepatocellular pattern ?
Elevated aminotransferases out of proportion to alkaline phosphatase
Cholestatic pattern?
Elevated alkaline phosphatase +gamma glutamyl transferase + bilirubin out of proportion to AST and ALT
AST vs ALT
AST is an amintransferase that is neither specific nor sensitive to liver disease as it is a cytosolic and mitochondrial isoenzyme found in many organs other than the liver. Whereas ALT is highly specific and sensitive to the hepatocellular injury.
Albumin in liver disease ?
relatively unaffected in acute injury and reduced in chronic liver disease.
Alkaline phosphate in biliary disease ?
ALP is a zinc metalloenzyme highly concentrated in the in the microvilli of the bile canaliculus. It is a marker of Biliary tree or duct obstruction.
Gama Glutamyltransferese significance ?
the elevation of the glycoprotein GGT is seen in hepatobiliary disease. Highly sensitive for high alcohol consumption and drug addiction.
clinical significance of testing pancreatic enzymes.
They are elevated in pancreatic injuries. Lipase is a more useful marker of acute pancreatitis as it has longer duration of elevation than amylase.
ESR
it is the rate at which blood cells settle in a test tube. It is a marker of chronic and acute inflammation.
C-reactive protein
CRP is elevated in acute and chronic inflammation. It is produced by the liver in response to acute inflammation.
Triglyceride, HDL, and Total cholesterol are ?
directly measured.
LDL is estimated using the formula
Total cholesterol - HDL- ( Triglyceride/5)
Non-HDL cholesterol is estimated by
Total cholesterol- HDL
Indication for blood culture ?
To detect bacteraemia ( transient, intermittent, continuous or sepsis and shock).
collection procedure of blood sample for culture in sepsis ?
- 3 samples at an interval of 30 to 60 min.
- ideally one sample before fever spike.
- Must be done before antibiotics started
- positive samples should be subcultured on Agar plates.
If two or more samples positive and
clinical presentation consistent with
infection ?
Treat identified organism based
on susceptibility
If one sample positive in blood culture ?
**Treat if organism is HIGH probability of true
positive AND clinical indications still present
*Don’t treat if organism is LOW probability
**If INTERMEDIATE probability, clinical
judgement and/or retesting
Nucleic Acid Amplification ?
is a highly sensitive laboratory technique to detect causative organism such as respiratory panel , HIV load , HCV, HSV etc.
Use of serology ?
Serology is used to detect host antibodies against the antigens of known pathogens.
Antigen detection
It is used to detect specific antigens + ca not distingusih viable from non viable.
Indication for prothrombin time ?
*Assess for suspected abnormalities of the extrinsic and final common pathway
*Monitor warfarin therapy
*Assess synthetic function of the liver (eg, in liver failure)
*Evaluation of unexplained bleeding
*Diagnose DIC
aPTT
*Assess for suspected abnormalities of the intrinsic and final common pathway
*Monitor heparin therapy (in patients on unfractionated heparin infusions)
*Assess synthetic function of the liver (eg, in liver failure)
*Evaluation of unexplained bleeding
*Diagnose DIC
How is INR calculated
Patients PT/ Control PT
Normal INR = 1
Normal INR on warfarin therapy
2.5 ( 2.0-3.0)
Normal eGFR
> 90
