Epi practicals - 12 April 2021

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12 April 2021

Is this test useful: Practical - HIV

  • Q1
With disease Without disease
test positive 152 20
test negative 8 980
  • Q2
    • Sensitivity=[math]\displaystyle{ \frac{152}{160} = 0.95 }[/math]
    • Specificity=[math]\displaystyle{ \frac{980}{1000} = 0.98 }[/math]
  • Q3

a)

With disease Without disease sum
test positive 38 1999.2 2037.2
test negative 2 97960.8 97962.8
sum 40 99,960 100,000
  • Positive predictive value=[math]\displaystyle{ \frac{38}{38+1999.2}=0.0187 }[/math]
  • Negative predictive value=[math]\displaystyle{ \frac{97960.8}{2+97960.8}=0.999 }[/math]

b)

With disease Without disease
test positive 316.35 13.34
test negative 16.65 653.66
sum 333 667
  • Positive predictive value=[math]\displaystyle{ \frac{316.35}{316.35+13.34}=0.959 }[/math]
  • Negative predictive value=[math]\displaystyle{ \frac{653.66}{16.65+653.66}=0.975 }[/math]
  • Q4

To have high sensitivity is more important, because testing blood donor is a screening test and false-negative has to be excluded as much as possible.

  • Q5

We need to calculate PPV and show the director how much false-positive donors arise.

  • Q6

We should advise the director that donors should be informed to proceed to an additional confirmation test, not just be informed simple positive result.

Is this test useful: COVID-19 Practical

  • Question 1
RT-PCR Positive RT-PCR Negative
IgG/M RDT Positive 91 6
IgG/M RDT Negative 7 174
sum 98 180
Sensitivity=[math]\displaystyle{ \frac{91}{98} = 92.9\% }[/math]
Specificity=[math]\displaystyle{ \frac{174}{180} = 96.7\% }[/math]
  • Question 2
    • We should know each figures of positive results of IgG and IgM, because IgG only appears during convalescent period after acute infection and is of little use for bedside diagnosis.
    • As well as we still don't know well when IgM appears in blood during acute phase of COVID, which means even IgM RDT may be of little use for bedside diagnosis.
    • One more thing we have to be careful is that RT-PCR would show positive for a long period after infection like 2-4 weeks, thus even though RDT shows good sensitivity and specificity compared to RT-PCR collected from asymptomatic persons, RDT results do not necessarily reflect acute phase status.
  • Discussion


  • Question 3
    • (a) No
      • Given the prevalence of RT-PCR positive results count 50% of asymptomatic HCWs, why antibody RDT might help to lift self-isolation of mildly symptomatic HCWs .... ? Icouldn't understand what this question wanted to ask.
    • (b) No
      • Because IgM/G RDT positive results cannot tell whether the patients are in acute or convalescent or post-infection status.
    • (c) Partially yes
      • If combination of negative IgM and positive IgG truly meant post-infection and guaranteed definite immunity, the RDT could be used for the purpose.
  • Question 4
    • We still could not have established definite gold or reference standard against acute infection of COVID, so I guess the only possible reference standard against new RT-PCR would be existing RT-PCR.