SpPin and SnNout

To understand what is meant by the terms SpPin and SnNout, we need to understand the notions of sensitivity and specificity.

SpPin

When a sign, test or symptom has an extremely high specificity (say, over 95%), a positive result tends to rule in the diagnosis. For example, the specificity of 3 or more positive responses on a CAGE questionnaire in diagnosing alcoholism is >99% among internal medicine patients. Therefore, if a person does answer “yes” to 3 or 4 of the CAGE questions, it rules in the diagnosis of alcohol dependency.

SnNout

When a sign, test or symptom has a high sensitivity, a negative result rules out the diagnosis. For example, the sensitivity of the loss of retinal vein pulsation in diagnosing high intracranial pressure is 100 per cent. Therefore, if a person displays retinal vein pulsation, it rules out important increases in intracranial pressure.

These terms are closely related to the measures of:

  • Positive Predictive Value: The proportion of people with a positive test who have the target disorder; and
  • Negative Predictive ValueThe proportion of people with a negative test who do not have the target disorder.

Often the best place to look for SpPins and SnNouts is at the highest (for SpPins) and lowest (for SnNouts) levels of multilevel likelihood ratios.

Calculations

These can be calculated thus:

sensitivity = a/(a+c)
specificity = d/(b+d)
likelihood ratio (LR+) = sensitivity / (1-specificity) = (a/(a+c)) / (b/(b+d))
likelihood ratio (LR-) = (1-sensitivity) / specificity = (c/(a+c)) / (d/(b+d))
positive predictive value = a/(a+b)
negative predictive value = d/(c+d)

Example

Table: Retinal veins, pulsation of, and increased intracranial pressure

HIGH
Intracranial Pressure
NORMAL
Intracranial Pressure
Total
ABSENCE of
retinal vein pulsation
43 18 61
PRESENCE of
retinal vein pulsation
0 128 128
total 43 146 189

HIGH by lumbar puncture (>190 mm H20), surgery, or evidence of herniation.
NORMAL by the absence of signs, symptoms, or suspicion of high pressure.

(Sensitivity and the loss of SRVP = 100% = SnNout!; presence of SRVP in normals = Specificity = 128/146=88%)

[Levin BE: The clinical significance of spontaneous pulsations of the retinal vein. Arch Neurol 1978;35:37-40]

Walsh et al checked this out by watching RVP during the Queckenstedt manoeuvre among a grab sample of 9 neurology patients who had normal pulsation prior to the LP:

  • disappeared when spinal fluid pressure rose past 204 (+/- 17) mm H2O
  • reappeared when spinal fluid pressure fell past 201 (+/- 17) mm H2O

[Walsh TJ, Garden JW, Gallacher B: Obliteration of retinal vein pulsations during elevation of cerebrospinal-fluid pressure. Amer J Opthalmology 1969;67:954-6.]

2 comments on “SpPin and SnNout

  1. Peter Tagmose Thomsen

    However, whether a test is good or not depends on the prevalence of disease in the population studied.
    i.e therefore test sensitivty / specificity from the hospital setting cannot readily be used in a private practise with a much lower disease prevalence

    Likelihood ratio’s take prevalence into account and would clinically speaking be a better tool

    Source: Rational Diagnosis and Treatment: Evidence-Based Clinical Decision-Making, P. G√łtzche et al, 2004

    • Minervation Post author

      Thanks Peter

      You are right – the populations may be quite different.

      LRs can be calculated from the Sensivitiy (Sn) and Specificity (Sp):
      LR for a positive result = sens / (1-spec)
      LR for a negative result = (1-sens) / (spec)

      More here

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