biochemical results for prescribing Flashcards
Why do biochemical tests ?
1- Diagnosis confirmed/substantiated
2- To assess severity of disease
3- To monitor response to therapy
4- To detect complications or adverse effects of therapy
5- To check adherence (?)
when is therapeutic drug monitoring useful
Narrow therapeutic window eg, theophylline
Poor correlation between dose and serum concentrations eg, Phenytoin
Wide inter-patient variability
when is therapeutic drug monitoring NOT useful
when pharmacological effects can be measured by other measures E.g. anti hypertensives and BP
Poor concentration-response relationship
which drugs are monitored ?
1- Drugs used prophylactically to maintain absence of condition
E.g. Seizures – Phenytoin, Cardiac arrhythmias - Digoxin, Organ rejection – Ciclosporin, Tacrolimus
2- Drugs which can cause serious toxicity
E.g. Vancomycin, Methotrexate (chemotherapy)
what are 5 indications to take drug levels ?
To check for adherence
To avoid toxicity (early after initiation)
To confirm toxicity
When clinical circumstances change acutely e.g. renal or hepatic failure
When drug interactions are likely
When should drug levels be taken ?
At steady-state (after about 5 half-lives)
Prior to next dose (except phenytoin – long half-life)
Lithium – 12 hours after dose
TDM is only one element in the overall patient management
What factors can cause variations in blood tests ?
Diet - High fat meals can affect TG levels
Drugs- e.g phenytoin rifampicin GGT
Time- e.g. diurnal variation in plasma cortisol
Menstrual cycle- e.g. plasma iron levels
Intense muscular exercise- e.g. increased plasma creatinine kinase and blood lactate levels
Dilution- e.g. Hb falling after resuscitation with crystalloids
Age- e.g. creatinine
Sex- Different reference levels
Race- Some differences reported for lipid profiles and plasma proteins
what are considerations when interpreting results within reference ranges?
Does each result fit in with previous assessments of the patients? - Trends/History/S+S
Has a significant change occurred?
Do any of the results alter the diagnosis or management of the patient?
What action needs to be taken?
reference range includes 95% of the population.
5% of healthy population will have “abnormal result”
what is sensitivity?
Sensitivity is the percentage of true positives
e.g. 90% sensitivity = 90% of people who have the target disease will test positive
What specificity /
Specificity is the percentage of true negatives
e.g. 90% specificity = 90% of people who do not have the target disease will test negative
What is Positive predictive value (PPV)
the probability that following a positive test result, that individual will truly have that specific disease.
PPV= True positive divided by (True positive + False positive)
what is Negative predictive value (NPV)
probability that following a negative test result, that individual will truly not have that specific disease.
NPV = True negative divided by (False negative + true negative)
which are some of the main biochemical tests ?
U+E
LFT
FBC
TFTs
