Epi practicals - 12 April 2021

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.