Thursday, 13 January 2011

Diagnostic Test Performance

Sensitivity: PIDPositive In Disease” = true positive / all who are really diseased (true positive + false negative by that test)

Specificity:  NIHNegative In Health” = true negative / all who are really disease free (true negative + false positive by that test)

Positive Predictive Value (PPV):  true positive / all +ve results by that test in other words true positive + false positive

Negative Predictive Value (NPV):  true negative / all –ve results by that test in other words true negative + false negative

In simple words:

Sensitivity: probability of a positive TEST result if the patient is diseased.

Specificity: probability of a negative TEST result if the patient is not diseased.

PPV: probability that a PATIENT tested positive is indeed diseased.

NPV: probability that a PATIENT tested negative is indeed not diseased.

The predictive value of a test not only depends on its sensitivity and specificity, but also on the disease prevalence.

The more prevalent the disease, the higher the positive predictive value of the test and the lower its negative predictive value and vice versa.

This may confuse some, as one might say, why on earth would the prevalence be affecting the whole issue in the population tested?

The following example will clarify it:

Using a test with a sensitivity of 100 % and a specificity of 99 % (i.e. 1 false positive in 100), and screening 1,000 patients, one would expect to see the following:

*In a setting with a high disease prevalence of 10 % (i.e. 100 per 1,000)
100 true positives per 1,000
10 false positives per 1,000 (constant)
Positive predictive value: 100 true positives/110 total positives = 91 %
100 (91 %) of the 110 positive test results are true.

*In a setting with a low disease prevalence of 0.1 % (i.e. 1 per 1,000)
1 true positive per 1,000 (this figure is now significantly lower as the disease is less prevalent)
10 false positives per 1,000 (constant)
Positive predictive value: 1 true positive/11 total positives = 9.1 %
Only 1 (9.1 %) out of 11 positive results is true.
In other words, >90 % of patients tested positive in a low-prevalence population will be actually false positives!

Saturday, 8 January 2011

Sexually Transmitted Infections (STI) testing update

Silver Jubilee ESDV Conference 
Marriott Hotel, Zamalek, Cairo, Egypt

Sexually Transmitted Infections (STI) testing update, January 2011.

Note that a test of cure is now recommended for ALL cases of gonorrhoea (culture should be delayed for at least 72 hours and NAAT at least 2 weeks after treatment).

This page was last updated in July 2011.

Management of Erectile Dysfunction in hepatic insufficiency

Egyptian Association for the Study of Gastrointestinal and Liver diseases - Special Interest Group Liver and systemic diseases Conference, Management of Erectile Dysfunction in hepatic insufficiency, October 2009.

HIV Vaccine and Vaccines in HIV infection

World AIDS Day (WAD) Conference, Kasr El Aini School of Medicine, Cairo University, HIV Vaccine and Vaccines in HIV infection, December 2007.

Sexually transmitted diseases pattern among men attending a specialised hospital (El-Haud El-Marsoud), thesis


My thesis dealt with different aspects of sociocultural and socioeconomic factors together with sexual orientation that interacted with sexual attitudes and practices of my patients at the Sexual Health Clinic, El Haud El Marsoud Hospital (
a hospital with walk-in clinics, for skin and venereal diseases in Cairo, established by the UK in 1902). The USA Government provided the hospital with a suitable STD laboratory at the time. 

Gohar A. Sexually transmitted diseases pattern among men attending a specialised hospital (El-Haud El-Marsoud). Cairo University, 1993.

Sexually transmitted urethritis was the commonest STD in my study.

Syphilis ranked the second among the diagnosed STDs in my study.

Anogential warts ranked the fourth among the diagnosted STDs in my study. 

Genital mollusca contagiosa ranked the sixth among the diagnosted STDs in my study.

Other relevant photos:

This page was last updated in February 2016

Main Works of Reference List (The first eight are my top favourites)

  • British National Formulary
  • British National Formulary for Children
  • Guidelines (BAD - BASHH - BHIVA - Uroweb)
  • Oxford Handbook of Genitourinary Medicine, HIV, and Sexual Health
  • Oxford Handbook of Medical Dermatology
  • Rook's Textbook of Dermatology
  • Simple Skin Surgery
  • Weedon's Skin Pathology
  • A Concise Atlas of Dermatopathology (P Mckee)
  • Andrews' Diseases of the Skin
  • Andrology (Nieschlag E FRCP, Behre M and Nieschlag S)
  • Bailey and Love's Short Practice of Surgery
  • Davidson's Essentials of Medicine
  • Davidson's Principles and Practice of Medicine
  • Fitzpatrick's Colour Atlas and Synopsis of Clinical Dermatology (Klaus Wolff FRCP and Richard Allen Johnson)
  • Fitzpatrick’s Dermatology in General Medicine
  • Ganong's Review of Medical Physiology
  • Gray's Anatomy
  • Hamilton Bailey's Demonstrations of Physical Signs in Clinical Surgery
  • Hutchison's Clinical Methods
  • Lever's Histopathology of the Skin
  • Lever's Histopathology of the Skin (Atlas and Synopsis)
  • Macleod's Clinical Examination
  • Martindale: The Complete Drug Reference
  • Oxford Handbook of Clinical Examination and Practical Skills
  • Oxford Textbook of Medicine
  • Practical Dermatopathology (R Rapini)
  • Sexually Transmitted Diseases (Holmes K et al)
  • Statistics in Clinical Practice (D Coggon FRCP)
  • Stockley's Drug Interactions
  • Treatment of Skin Disease: Comprehensive Therapeutic Strategies
  • Yen & Jaffe's Reproductive Endocrinology