usmle: equations gone wild
Terms
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- Sensitivity
-
TP/(TP+FN)
Probability that a person having a disease will be correctly identified - Specificity
-
TN/(TN+FP)
Probability that a person who doesn't have a disease will be correctly identified - Positive-Predictive Value
-
TP/(TP+FP)
Probability that an individual who tests (+) has the disease - Negative-Predictive Value
-
TN/(TN+FN)
Probability that a person who tests (-) doesn't have the disease - Prevalence
-
TP+FN/(total # in study)
Total number of cases in a population at a given time - Incidence
-
# new cases each year/*susceptible* population
(ex: In a population of 100, there are 10 new cases of cat scratch fever every year. The prevalence of CSF in this population is 20. Therefore, the *incidence* of CSF in this population is 10/80) - Relative Risk
-
Used in cohort studies.
(TP/TP+FP)/(FN/FN+TN) - Odd Ratio
-
Used in case-control studies
(TP x TN)/(FPxFN) - Mean
- average
- Median
- Middle value in sequentially ordered groups of #'s
- Mode
- # that appears most often in a group
- Positive skew
-
peak is to the left
mean>median>mode - Negative skew
-
peak is to the right
mean<median<mode - When prevalence decreases, sensitivity and specificity are...
- unchanged
- When prevalence decreases, what happend to PPV and NPV?
- PPV decreases and NPV increases
- Hardy-Weinberg population genetics equation
-
If "p" and "q" are different alleles, then:
p2+2pq+q2=1
or
p+q=1 -
Heterozygote frequency for
H-W population genetics - HF=2pq
- Henderson-Hasselbalch
- pH=pKa + log[A]/[HA]
- How to convert logs
-
log (base 10) a = x
or
10x = a - Volume of distribution
- Vd = (amount of drug in body)/(plasma drug concentration)
- Drug clearance (1 of 4)
- CL = (rate of drug elimination)/(plasma concentration of drug)
- Drug clearance (2 of 4)
- CL = Ke x Vd
- Drug clearance (3 of 4)
- CL = (0.7 x Vd)/t1/2
- Drug clearance (4 of 4)
- CL = Infusion rate/(steady state concentration)
- Half life (1 of 2)
- t1/2 = (0.7 x Vd)/CL
- Half life (2 of 2)
- t1/2 = 0.7/Ke
- Therapeutic index
- TI = LD50/ED50
- Loading dose
-
LD = (Cp x Vd)/F
F=bioavailability - Maintainence dose
- MD = (Cp x CL)/F
- Infusion rate
- IR = CL x steady state conc.
- Cardiac output (1 of 3)
- CO = (rate of O2 consumption)/(arterial O2 content - venous O2 content)
- Cardiac output (2 of 3)
- CO = SV x HR
- Cardiac output (3 of 3)
- CO = MAP/TPR
- Mean arterial pressure (1 of 2)
- MAP = CO x TPR
- Mean arterial pressire (2 of 2)
- MAP = 1/2 systolic + 2/3 diastolic
- Stroke volume (1 of 2)
- SV = CO/HR
- Stroke volume (2 of 2)
- SV = end diastolic volume - end systolic volume
- Ejection fraction
- EF = (SV/EDV) x 100
- Glomerular filtration rate (1 of 2)
- GFR = (Uinulin x V)/(Pinulin)
- Glomerular filtration rate (2 of 2)
- GFR = FF x RPF
- GFR is equal to clearance of *what* and is close enough to the clearance of *what else*?
-
GFR = inulin clearance
GFR ~ creatinine clearance - Effective renal plasma flow
- ERPF = (Upah x V)/(Ppah)
- Effective renal plasma flow is equal to the clearance of *what*?
- ERPF = pah clearance
- Renal blood flow
- RBF = RPF/(1-Hct)
- Renal plasma flow (1 of 2)
- RPF = RBF x (1-Hct)
- Renal plasma flow (2 of 2)
- RPF = GFR/FF
- Free water clearance
- Ch2o = V - Cosm
- How do you get total body water from total body weight?
- total body water is generally 60% of a man's total body weight and 50% of a woman's
- How do you measure intracellular fluid (ICF)?
-
ICF = TBW - extracellular fluid (ECF)
ICF is about 2/3 TBW - How do you measure ECF?
- inulin, mannitol, sulfate
- How do you measure plasma?
-
It's about 1/4 ECF
(or, whatever was measured with evan's blue) - How do you measure interstitial fluid?
-
IF = ECF - plasma volume
IF is about 3/4 the ECF - Vital capacity
- VC = TV + IRV + ERV
- Functional reserve capacity
- FRC = RV + ERV
- Inspiratory capacity
- IC = IRV + TV
- Total lung capacity
- TLC = IRV + TV + ERV + RV
- i love equations gone Wild!
- yee haw!
- confidence interval of the mean
-
CI = mean +/- Z(S/sqrt(N))
For a 95% CI, Z=1.96. Make it 2 for convenience's sake. - Series resistance
- Rt = R1+R2+R3...
- Parallel resistance
- 1/Rt = 1/R1+1/R2+1/R3...