Using numbers and handling data

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# Questions relating to Figure 11

The following OD450 values were measured from serum samples taken from three babies: 0.12, 0.40, and 1.74.

## SAQ 7

Use the calibration curve shown in Figure 11 to estimate the serum concentration of CRP for each sample.

From the curve you can deduce the values are about 2 g/ml, 7 g/ml, and 90 g/ml, respectively.

## SAQ 8

Does the existing scale allow you to confidently estimate these values?

You probably found it difficult to make accurate CRP readings for the OD450 values of 0.12 and 1.74. This is because the scale of the y-axis (the OD450 values) is too compressed. If the graph were redrawn, with the y-axis maybe twice its current height (but still running from 0 to 2) then this would provide a better resolution.

## SAQ 9

One of your estimated concentrations might be particularly inaccurate; which one do you think this is likely to be and why?

Apart from the problem already mentioned concerning the size of the y-axis, the OD450 measurement of 1.74 is likely to be inaccurate. This is because the measuring assay is becoming saturated, as you can see by the calibration curve flattening off towards a constant high value in this region. The colour of the test solution is almost at its maximum intensity and changes in CRP concentration make almost no difference to the OD450 reading.

## SAQ 10

What do you infer about the health of each baby from these results?

Earlier on in this section you were told what level of infection corresponded with a particular range of serum CRP concentrations:

normal: <2 μg/ml

sepsis: >22 μg/ml

dying: >120 μg/ml

You have calculated the CRP values of three babies to be about 2 μg/ml, 7 μg/ml, and 90 μg/ml. Using these figures for reference, you can see that the baby with a serum CRP concentration of 2 μg/ml is healthy, although at the high end of the range for a healthy baby. The baby with a CRP concentration of 7 μg/ml is beyond the normal level and would be expected to have a mild fever, but is not yet dangerously unwell. The baby with a CRP concentration of 90 μg/ml is very seriously ill and will require intensive care.

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